Article-At-A-Glance: What Every Rare Gold Coin Investor Should Know

• Rare gold coins carry two layers of value — their gold content and their numismatic (collector) value — which can make them far more profitable than standard bullion.
• Pre-1933 U.S. Gold coins are among the most sought-after rare gold coins due to a fixed and finite supply that will never increase.
• Always buy coins graded by the Numismatic Guaranty Company (NGC) or the Professional Coin Grading Service (PCGS) to protect yourself from counterfeits.
• The adage “buy the book before the coin” holds true — understanding numismatics before spending a dollar is the single best move a new investor can make.
• Keep reading to discover which specific rare gold coins have proven track records, and what red flags to watch out for when buying.

Most gold investors are leaving serious money on the table by ignoring the numismatic side of the market.

Gold bullion is straightforward — you buy it, it tracks the spot price, and you hold it as a hedge. Rare gold coins work differently. Their value is shaped by rarity, historical significance, condition, and collector demand, all of which can push a coin’s price far beyond what the gold alone is worth. That dual-value structure is exactly what makes rare gold coins one of the most compelling asset classes available to serious investors. Austin Rare Coins & Bullion has been helping investors navigate this market since 1989, and the insight they’ve built over decades reflects just how nuanced and rewarding this space can be.

Understanding the difference between buying gold and buying rare gold coins is the first step toward building a collection that actually grows in value over time.

Rare Gold Coins Are Worth Far More Than Their Weight in Gold

A standard one-ounce gold bullion coin is worth roughly whatever the current spot price of gold is on any given day. A rare gold coin from the same era — same weight, same purity — might sell for five, ten, or even one hundred times that amount. The difference comes down to everything that surrounds the gold itself.

Take the 1933 Saint-Gaudens Double Eagle as an extreme example. The coin contains less than one ounce of gold, yet a single specimen sold at auction for over $18 million. That price has nothing to do with gold’s spot price. It has everything to do with rarity, history, and collector demand converging on a single object.

Even more accessible rare gold coins — Pre-1933 U.S. Double Eagles, British Gold Sovereigns, or Swiss Gold Francs — consistently trade at meaningful premiums above their melt value. For investors who take the time to understand the market, that premium is not a cost. It is an opportunity.

What Makes a Gold Coin “Rare” and Valuable

Not every old gold coin is rare, and not every rare gold coin is a great investment. Four key factors drive value in this market, and understanding each one will sharpen your ability to spot genuine opportunities.

Rarity and Mintage Numbers

Mintage refers to how many coins were originally struck. Lower mintage numbers generally translate to higher rarity and, over time, higher demand. Pre-1933 U.S. Gold coins are a textbook example — after President Roosevelt’s 1934 recall order required citizens to turn in their gold coins, the surviving population of these coins became permanently and irreversibly limited. No new ones will ever enter the market, which means supply can only shrink as coins are lost, damaged, or locked away in private collections.

Coin Condition and Grading Standards

Two coins with identical mintage numbers can have vastly different values based solely on their condition. The numismatic world uses the Sheldon scale, a 70-point grading system, to assess a coin’s physical state. A coin graded MS-65 (Mint State) commands a dramatically higher price than the same coin graded VF-30 (Very Fine). Even minor differences in grade can mean thousands of dollars in value, which is why independent grading is non-negotiable before any significant purchase.

When evaluating two similar coins, always favor the one with better eye appeal — strike sharpness, luster, and surface quality all factor into desirability beyond the technical grade alone.

Historical Significance and Age

Coins tied to significant historical moments carry an additional layer of collector demand that purely modern coins cannot replicate. Ancient Roman gold coins, early American gold pieces, and coins from landmark historical periods attract both numismatists and history enthusiasts, broadening the base of potential buyers when it comes time to sell.

Age alone does not guarantee value — a common-date ancient coin may be worth less than a low-mintage modern rarity. But historical context consistently adds a premium that survives market fluctuations.

Market Demand and Collector Interest

Rarity means little without demand. The most valuable rare gold coins sit at the intersection of limited supply and strong, sustained collector interest. Monitoring auction results, dealer pricing trends, and collector forums gives you a real-time read on where demand is moving — and which coins are falling out of fashion.

Numismatic Value vs. Bullion Value: What Is the Difference

Every gold coin has a floor value — its melt value, determined by weight and gold purity. Numismatic value is everything above that floor, driven by the factors described above. Understanding where a specific coin sits on that spectrum is critical before committing capital.

Why Numismatic Value Can Dwarf the Spot Price of Gold

When a coin is scarce, historically significant, and in exceptional condition, collectors will pay whatever it takes to acquire it. That competitive dynamic detaches the coin’s price from gold’s spot price entirely. A well-chosen rare coin purchased at the right grade and the right moment can appreciate far faster than gold bullion ever could.

When Bullion Value Matters More Than Numismatic Value

Common-date coins in lower grades may carry only a modest premium above melt value. In those cases, the coin behaves more like bullion than a collectible, and its price will rise and fall closely with gold’s spot price. This is not necessarily bad — it simply means you are buying gold with a coin shape rather than a rare numismatic asset. For those interested in purchasing bullion, JM Bullion is a popular choice for buying gold coins.

Knowing which category your target coin falls into before you buy is what separates informed investors from those who overpay expecting numismatic appreciation that never materializes. For a comprehensive guide on coin investment, consider reviewing the Noble Gold Investments guide.

The Best Rare Gold Coins to Invest In

These are not obscure picks or speculative long shots. Each of the following coin categories has a proven track record of collector demand, market liquidity, and long-term value retention. The best entry point depends on your budget, your goals, and how deeply you want to engage with the numismatic side of the market.

1. Pre-1933 U.S. Gold Double Eagles

The $20 Gold Double Eagle is the crown jewel of American numismatics for most serious investors. Struck between 1849 and 1933, these coins contain 0.9675 troy ounces of gold and combine genuine precious metal content with a permanently fixed and declining supply. The two most iconic designs — the Liberty Head (Coronet) and the Saint-Gaudens — are widely recognized, aggressively collected, and consistently liquid in the secondary market.

What makes Pre-1933 Gold particularly compelling is the combination of finite supply and broad demand. Roosevelt’s 1934 recall removed enormous quantities from circulation permanently, meaning every surviving example is genuinely scarce. Common-date Double Eagles in circulated grades represent an accessible entry point, while key-date specimens in high Mint State grades are portfolio-defining acquisitions for advanced collectors.

2. Gold American Eagle Coins

First struck in 1986, the Gold American Eagle is the United States’ official gold bullion coin, but certain dates and mint marks have crossed into genuine numismatic territory. The 1999-W Unfinished Proof Dies and the 2008-W Burnished coins are prime examples of modern Gold Eagles that command significant premiums due to low mintage and collector demand. They offer a bridge between bullion investing and numismatic collecting — making them an ideal starting point for investors not yet ready to commit to Pre-1933 coins.

3. Swiss Gold Francs

The Swiss 20 Franc Helvetia, minted between 1897 and 1949, contains 0.1867 troy ounces of gold and trades at modest premiums above spot in most market conditions. These coins are highly regarded for their consistent quality, attractive design, and broad international recognition. For investors looking to diversify across geographic origins, Swiss Gold Francs offer a stable, liquid option with genuine historical character.

4. British Gold Sovereigns

The British Gold Sovereign has been struck continuously since 1817, making it one of the most recognized and trusted gold coins in the world. Each Sovereign contains 0.2354 troy ounces of 22-karat gold and features the iconic St. George slaying the dragon reverse design. Certain dates and reign types — particularly Victorian-era Sovereigns and early Edward VII issues — command strong numismatic premiums, while common-date examples remain one of the most liquid gold coins available globally. For investors who value both history and ease of resale, the British Gold Sovereign is a near-perfect holding.

5. Ancient Roman Gold Coins

Ancient Roman gold coins, known as Aurei (singular: Aureus), were struck from approximately 46 BC through the late Roman period and represent one of the oldest investable gold coin categories in existence. Each Aureus was minted to a high standard of purity and typically featured the portrait of the reigning emperor on the obverse, making individual coins historically traceable to specific rulers and periods in Roman history.

The appeal here goes well beyond gold content. Owning a coin that circulated during the reign of Julius Caesar, Augustus, or Marcus Aurelius connects the investor to over two thousand years of recorded history. Demand comes from both serious numismatists and museum-grade collectors, creating a deep and resilient buyer base. Prices range from a few thousand dollars for common emperors in lower grades to six figures for rare rulers in exceptional condition. Authenticity verification from a trusted specialist is absolutely essential before purchasing any ancient coin.

6. China Gold Panda Coins

First issued by the People’s Bank of China in 1982, the Gold Panda series is one of the most visually distinctive and internationally collected modern gold coin programs in the world. What sets these coins apart is a design feature unique among major sovereign gold coins — the panda image changes every single year, which means every annual issue is a limited-edition collectible by design. This built-in scarcity mechanism drives consistent collector demand year after year.

Early-year issues from the 1980s and early 1990s, particularly in Proof condition, have appreciated substantially above their original issue prices. The coins are produced in fractional sizes ranging from 1 gram to 30 grams, making them accessible at virtually every investment level. For investors interested in modern rare gold coins with genuine numismatic upside, the China Gold Panda series deserves serious attention.

How to Grade and Authenticate Rare Gold Coins

Grade determines value more than almost any other single factor in rare gold coin investing. A one-point difference on the Sheldon scale can represent a price difference of thousands of dollars on a desirable coin, which means understanding how grading works is not optional — it is foundational to protecting your investment.

The Sheldon Grading Scale Explained Simply

The Sheldon scale runs from 1 to 70, where 1 represents a coin that is barely identifiable and 70 represents a perfect, flawless specimen. The scale is divided into broad condition categories that every investor should understand before evaluating any coin:

• Poor (P-1) to About Good (AG-3): Heavily worn, design barely visible, value near melt only.
• Good (G-4 to G-6): Major design elements visible but flat, significant wear throughout.
• Very Fine (VF-20 to VF-35): Moderate wear on high points, all major details sharp.
• Extremely Fine (EF-40 to EF-45): Light wear on highest points only, strong overall detail.
• About Uncirculated (AU-50 to AU-58): Slight wear on the very highest points, most luster intact.
• Mint State (MS-60 to MS-70): No wear whatsoever, graded on quality of strike, luster, and surface preservation.

For rare gold coins, the Mint State range is where numismatic premiums accelerate dramatically. An MS-63 coin might sell for twice the price of the same coin in MS-61, and an MS-65 specimen can command multiples of that again. Eye appeal — the overall visual impact of the coin — plays a significant role within each grade tier and should never be ignored when comparing two coins at the same grade level.

Third-Party Grading Services Worth Trusting

Never rely solely on a seller’s own grading assessment for any significant rare gold coin purchase. The two most respected independent grading services in the industry are the Professional Coin Grading Service (PCGS) and the Numismatic Guaranty Company (NGC). Both services encapsulate graded coins in tamper-evident holders — commonly called “slabs” — with the grade, coin identity, and a certification number that can be verified online. A PCGS or NGC-certified coin trades with greater confidence, greater liquidity, and consistently stronger prices than raw (ungraded) examples of the same coin.

Red Flags That Signal a Counterfeit Coin

The rare gold coin market attracts sophisticated counterfeiters, and the higher the coin’s value, the greater the incentive to fake it. Watch for these warning signs before committing to any purchase:

• Weight or diameter that does not match official mint specifications precisely.
• Unusual color, luster, or surface texture inconsistent with genuine gold coins of that era.
• Seams, tool marks, or casting bubbles visible under magnification — signs of a cast fake rather than a struck coin.
• Prices dramatically below fair market value with no credible explanation.
• Sellers who resist independent authentication or third-party grading before sale.
• Packaging or holders that mimic PCGS or NGC slabs but lack a verifiable certification number.

How to Build a Rare Gold Coin Investment Strategy

Buying rare gold coins without a clear strategy is the fastest way to overpay, accumulate the wrong assets, and struggle when it comes time to sell. The investors who do well in this market treat it like any other serious asset class — with defined goals, realistic budgets, and a long-term horizon.

The numismatic market rewards patience and penalizes impatience. Unlike stocks, rare gold coins do not reprice every second. Opportunities emerge slowly, and the best purchases are almost always made by investors who waited for the right coin at the right grade rather than rushing into a position.

Set Clear Investment Goals Before You Buy

Are you buying rare gold coins primarily as a hedge against inflation and currency debasement? Are you targeting numismatic appreciation above and beyond gold’s spot price performance? Or are you building a collection with both financial and personal enjoyment in mind? Each goal points toward a different type of coin and a different buying strategy. For a comprehensive understanding of different investment strategies, you can explore Noble Gold Investments reviews.

Inflation hedge investors should lean toward coins with strong intrinsic gold content and broad market recognition — Pre-1933 Double Eagles and British Gold Sovereigns are natural fits. Numismatic appreciation seekers need to focus on key dates, low-mintage issues, and high-grade specimens where collector demand creates price performance independent of gold’s spot price. For more information on investing in precious metals, check out these precious metals IRA reviews.

How to Set a Realistic Budget for Rare Coins

Entry-level rare gold coins — common-date Pre-1933 pieces in circulated grades or Swiss Gold Francs — can be acquired for a few hundred to a few thousand dollars per coin. Mid-tier numismatic pieces in higher grades typically range from $2,000 to $25,000. True key-date rarities in premium Mint State grades regularly trade above $50,000 and into six figures. For more insights on investing in rare gold coins, check out the Lear Capital Gold Reviews.

A practical approach for new investors is to establish a per-coin budget ceiling and refuse to exceed it regardless of dealer enthusiasm or auction momentum. Emotional buying is the number one portfolio destroyer in this market. Define your range before you start looking, and stick to it until your knowledge and confidence grow enough to justify expanding it.

Diversify Across Coin Types and Eras

Just as a stock portfolio benefits from diversification across sectors, a rare gold coin portfolio benefits from diversification across coin types, countries of origin, and historical eras. Holding a mix of Pre-1933 U.S. Gold, British Sovereigns, and select modern rarities like early China Gold Pandas creates exposure to different collector bases, different demand drivers, and different liquidity profiles — reducing the risk that weakness in one segment undermines the entire portfolio.

Patience Is the Most Underrated Investment Tool

The rare gold coin market is not designed for quick flips. Dealers maintain buy-sell spreads, auction houses charge buyer’s premiums, and numismatic value builds over years and decades rather than weeks. The investors who consistently outperform in this space are those who buy quality coins at fair prices and hold them long enough for the market to fully recognize their value. If you need liquidity within twelve months, rare gold coins are the wrong asset class for that capital.

Where to Buy Rare Gold Coins Safely

The source of your rare gold coin purchase matters as much as the coin itself. Buying from the wrong place can expose you to counterfeits, overgraded coins, and prices that leave no room for future appreciation. There are three primary channels worth understanding: reputable dealers, auction houses, and coin shows — and each has its own advantages depending on where you are in your investment journey.

Reputable Dealers vs. Auction Houses: Pros and Cons

Established rare coin dealers offer consistency, expertise, and the ability to build an ongoing relationship that benefits you over time. A trusted dealer will tell you when a coin is overpriced, steer you toward better value, and often give returning clients access to inventory before it hits the open market. The trade-off is that dealer pricing includes a margin, so you are paying for that expertise and convenience. When evaluating any dealer, look for membership in the Professional Numismatists Guild (PNG) or the American Numismatic Association (ANA) — both organizations hold members to strict ethical standards. For more insights, you can explore collecting rare coins.

Auction houses like Heritage Auctions and Stack’s Bowers offer access to exceptional coins that rarely appear through dealer channels, and competitive bidding can occasionally produce fair prices on desirable pieces. The downside is real: buyer’s premiums typically run between 15% and 20% on top of the hammer price, and auction momentum can push prices well above fair market value when two determined bidders compete for the same coin. Auctions reward patience and discipline — set a maximum bid before the auction opens and treat it as an absolute ceiling, not a suggestion.

Coin Shows as a Learning and Buying Resource

Coin shows are one of the most underutilized resources available to rare gold coin investors, particularly those who are still building their knowledge base. Walking the floor of a major show like the ANA World’s Fair of Money exposes you to dozens of reputable dealers simultaneously, giving you a real-time education in pricing, condition, and what strong coins actually look like in hand. You can compare similar coins side by side, ask questions without any purchase pressure, and develop the visual literacy that no book or online forum can fully replace. Many experienced investors made their most important dealer relationships at coin shows, and those relationships paid dividends for decades.

How to Store and Protect Your Rare Gold Coin Collection

Rare gold coins are physically vulnerable in ways that stocks and bonds simply are not. Improper storage can cause irreversible damage — fingerprints, humidity, chemical reactions, and even the wrong type of holder can permanently reduce a coin’s grade and destroy thousands of dollars in numismatic value. The standard of care required for a rare coin collection is meaningfully higher than for standard bullion.

PCGS and NGC-graded coins in their original slabs offer the best protection for high-value pieces — the tamper-evident holder isolates the coin from environmental exposure and physical contact. For unslabbed coins, use inert, non-PVC flips or hard plastic capsules specifically designed for numismatic storage. Never use soft vinyl flips, which contain PVC that leaches onto gold surfaces over time and creates green or hazy film that is impossible to reverse. Store your collection in a fireproof, humidity-controlled safe, and consider a bank safe deposit box for your most valuable pieces. For insurance purposes, maintain a detailed photographic inventory with PCGS or NGC certification numbers for every significant coin in your collection.

The Risks Every Rare Gold Coin Investor Must Accept

Rare gold coins are a compelling asset class, but they are not without meaningful risks. Walking into this market with an honest understanding of the downside scenarios is not pessimism — it is the foundation of sound portfolio management. Every rare gold coin investor should understand these risk factors before committing serious capital. For those looking to diversify their portfolio, exploring options like a precious metals IRA might be a prudent choice.

Price Fluctuation Based on Spot Prices and Market Demand

Even coins with strong numismatic premiums are not completely immune to gold’s spot price movements. When gold prices fall sharply, the floor value of every gold coin drops with it, and some of that decline can pull numismatic premiums down as well — particularly for coins in lower grades where the numismatic premium above melt value is thin. High-grade key-date coins tend to be more insulated from spot price swings because their value is driven primarily by collector demand rather than metal content, but no rare gold coin is entirely disconnected from the broader precious metals market.

Market demand for specific coin types also shifts over time. A series that commands strong collector interest today may fall out of fashion as a new generation of collectors develops different preferences. Staying current with auction results, dealer price guides like the PCGS Price Guide and NGC Price Guide, and collector community discussions gives you the early warning signals needed to reposition before a demand shift erodes your portfolio’s value.

Liquidity Challenges With Highly Specialized Coins

The more specialized and high-value the coin, the smaller the pool of qualified buyers. A common-date Pre-1933 Double Eagle in circulated condition can be sold quickly to dozens of dealers or through multiple auction platforms with minimal friction. A six-figure key-date rarity in top Mint State grade requires finding the right buyer at the right moment, which can take months. Forcing a sale of a highly specialized coin under time pressure almost always means accepting a price below fair market value.

This liquidity gradient should directly influence how you build your portfolio. New investors are better served by coins with broad market recognition and a deep buyer base — coins that can be converted to cash within days rather than months. As your knowledge and financial position grow, allocating a portion of capital to rarer, higher-value specimens makes sense, but that portion should never represent money you might need access to quickly.

Start Small, Think Long-Term, and Buy What You Love

The investors who build the most successful rare gold coin portfolios share one trait above all others — they genuinely love what they own. That emotional connection drives deeper research, more disciplined buying decisions, and the patience to hold through market fluctuations without panic. Start with one or two well-chosen coins in a category that genuinely interests you, learn everything you can about that series, and let your knowledge compound alongside your collection. The financial results, for those who approach this market with seriousness and patience, have historically been exceptional.

Frequently Asked Questions

These are the questions most new rare gold coin investors ask first — and getting clear answers before making your first purchase will save you from the most common and costly mistakes in this market.

• What is the difference between a rare gold coin and a gold bullion coin?
• How much do rare gold coins typically cost?
• Are rare gold coins a good hedge against inflation?
• How do I know if a rare gold coin is authentic?
• Can I include rare gold coins in a retirement portfolio?

What Is the Difference Between a Rare Gold Coin and a Gold Bullion Coin?

A gold bullion coin — like a modern Gold American Eagle or a Canadian Gold Maple Leaf — is produced in large quantities specifically for investors who want exposure to gold’s spot price. Its value tracks gold almost directly, with a small premium for minting and distribution. A rare gold coin carries numismatic value above and beyond its gold content, driven by rarity, historical significance, condition, and collector demand. The best rare gold coins offer both a gold price floor and meaningful numismatic upside that bullion coins simply cannot provide. For more insights on gold investments, you can check out this JM Bullion review.

How Much Do Rare Gold Coins Typically Cost?

Entry-level rare gold coins — such as common-date Pre-1933 Liberty Head Double Eagles in circulated grades or Swiss 20 Franc Helvetia coins — typically start in the range of $500 to $3,000. Mid-tier numismatic coins in higher grades range from $3,000 to $25,000. Key-date rarities and high-grade Mint State specimens regularly trade from $25,000 into six figures, with the rarest pieces reaching into the millions at major auction houses. There is a genuine entry point at nearly every budget level, which makes this market more accessible than many new investors initially assume. For more insights, you might want to check out this JM Bullion review.

Are Rare Gold Coins a Good Hedge Against Inflation?

Yes — and for two distinct reasons that standard bullion cannot match. First, the gold content itself provides the same inflation protection that any physical gold holding delivers. Second, the numismatic premium on rare gold coins has historically grown during periods of economic uncertainty, as collectors and investors simultaneously compete for a fixed supply of coins. Pre-1933 U.S. Gold coins in particular have demonstrated long-term value appreciation that has outpaced both inflation and gold’s raw price performance over extended holding periods.

How Do I Know If a Rare Gold Coin Is Authentic?

The single most reliable protection against counterfeits is purchasing only PCGS or NGC-certified coins in their original, sealed holders. Both services maintain online verification databases — PCGS CoinFacts and NGC Coin Explorer — where any certification number can be confirmed in seconds. For raw (ungraded) coins, have any significant purchase authenticated by PCGS or NGC before completing the transaction. No reputable seller will object to this condition.

Beyond certification, there are physical characteristics worth understanding when evaluating any coin:

• Verify weight and diameter against official mint specifications using a precision scale and calipers.
• Examine the coin under magnification for casting seams, tool marks, or surface irregularities inconsistent with struck coinage.
• Assess the coin’s luster and color — genuine gold coins have a distinctive warmth and reflectivity that most counterfeits fail to replicate convincingly.
• Use a rare earth magnet — genuine gold is non-magnetic, and any attraction indicates base metal content.
• Consult a specialist for ancient coins or high-value rarities where the stakes of an authentication error are highest.

For any coin above $1,000 in value, independent third-party grading is not an optional precaution — it is the minimum standard of due diligence. The cost of PCGS or NGC grading is a small fraction of the protection it provides against overpaying for a counterfeit or overgraded coin.

Trust your instincts as well. If a coin’s price seems dramatically lower than comparable certified examples, or if a seller resists any form of independent verification, walk away. In the rare gold coin market, deals that seem too good to be true virtually always are.

Can I Include Rare Gold Coins in a Retirement Portfolio?

Yes, under specific conditions. The IRS allows certain gold coins to be held within a self-directed Individual Retirement Account (IRA), but the rules are strict. Coins must meet minimum purity standards — generally 99.5% pure gold — which excludes most Pre-1933 U.S. Gold coins (which are 90% pure) and many other numismatic pieces. Gold American Eagles are a notable exception explicitly permitted by the IRS despite being 91.67% pure.

For investors who want rare gold coin exposure within a tax-advantaged retirement account, the path requires working with a custodian that specializes in self-directed IRAs with precious metals holdings. The coins must be stored in an approved depository — you cannot take personal possession of IRA-held coins without triggering a taxable distribution. Setup and annual custody fees add to the cost structure, which must be weighed against the tax advantages for your specific financial situation.

Outside of retirement accounts, rare gold coins are held as tangible personal property and taxed as collectibles when sold, with a maximum federal capital gains rate of 28% — higher than the standard long-term capital gains rate that applies to most other investments. Understanding the tax implications of your specific coin holdings before buying or selling is an essential part of managing this asset class responsibly. Consulting a tax advisor with experience in collectible assets will help you structure your rare gold coin investments in the most tax-efficient way possible.

If you are ready to take your first step into rare gold coin investing with confidence, Austin Rare Coins & Bullion offers decades of expertise and a curated selection of investment-grade coins to help you build a collection worth owning. For additional insights, check out this Noble Gold Investments guide.

Investing in rare gold coins can be a lucrative endeavor for those looking to diversify their portfolio with tangible assets. However, it’s important to conduct thorough research and understand the market dynamics before making a purchase. For those interested in learning more about the investment process, this Noble Gold Investments guide offers valuable insights and reviews to help you make informed decisions.