Why Purity Matters
Gold purity determines both the value and the physical characteristics of a gold product. A 1 oz coin at .9999 fine contains 31.1035 grams of pure gold. A 1 oz coin at .9167 fine contains 28.52 grams of pure gold within a heavier coin (33.93 grams total). The pricing reflects the pure gold content, not the total weight.
For investment purposes, purity affects IRA eligibility, international recognition, appearance, and durability. Understanding the purity system prevents overpaying for lower-purity products and clarifies why different coins contain different alloy compositions.
The Karat System
Karats measure gold purity on a scale of 24. Pure gold is 24 karat (24K). Each karat represents 1/24th of the total composition by weight.
| Karat | Gold Percentage | Millesimal Fineness | Common Uses |
|---|---|---|---|
| 24K | 99.9%+ | .999 or .9999 | Investment bars, Maple Leaf, Philharmonic |
| 22K | 91.67% | .9167 | American Eagle, Krugerrand, British Sovereign |
| 18K | 75.0% | .750 | Fine jewelry |
| 14K | 58.3% | .583 | Standard US jewelry |
| 10K | 41.7% | .417 | Budget jewelry (minimum for “gold” in US) |
The karat system is intuitive: 24K means 24 out of 24 parts are gold. 18K means 18 out of 24 parts are gold (75%). 14K means 14 out of 24 parts (58.3%). The system originated in medieval Europe and remains the standard for describing gold content in consumer products.
In the US, the minimum karat for a product to legally be called “gold” is 10K (41.7% gold). Below that threshold, the item cannot be marketed as gold.
Millesimal Fineness
The millesimal fineness system expresses purity as parts per thousand. It is the standard in the bullion industry and for international trade.
| Fineness | Meaning | Examples |
|---|---|---|
| .999 | 999 parts gold per 1,000 | Most gold bars, some coins |
| .9999 | 9,999 parts gold per 10,000 | Canadian Maple Leaf, Perth Mint products |
| .99999 | 99,999 parts gold per 100,000 | Royal Canadian Mint specialty coins |
| .9167 | 916.7 parts gold per 1,000 | American Eagle, Krugerrand |
| .750 | 750 parts gold per 1,000 | 18K jewelry |
The difference between .999 and .9999 is small in absolute terms: one extra part per ten thousand. In a 1 oz bar, the difference in gold content is approximately 0.031 grams, worth roughly $2.50 at current prices. The practical investment difference is negligible.
The difference between .999 and .9167 is far more significant. A 1 oz American Eagle at .9167 fineness requires 33.93 grams of total metal to deliver 31.10 grams of gold. The remaining 2.83 grams are silver and copper alloy. The buyer pays for the gold content; the alloy metals add negligible value but do add weight.
How Gold Purity Is Measured
Three primary methods are used to determine gold purity, each with different precision, cost, and destructiveness.
Fire Assay (Cupellation)
The most accurate method, capable of measuring purity to 0.01%. A small sample is melted with lead in a cupel (porous cup). The lead absorbs base metals, leaving pure gold. The pure gold is weighed and compared to the original sample weight.
Fire assay is destructive: the sample is consumed in the process. It is used for refinery quality control, dispute resolution, and as the reference standard against which other methods are calibrated. Not practical for testing individual coins or bars.
X-Ray Fluorescence (XRF)
XRF analyzers fire X-rays at a gold surface and measure the characteristic fluorescence emitted by different elements. The resulting spectrum identifies the elements present and their approximate concentrations. Results are available in seconds.
XRF is non-destructive and widely used by dealers and assay offices. Handheld units cost $15,000-30,000. Limitations: XRF measures surface composition only (penetration depth of roughly 10-50 microns). A gold-plated tungsten bar could pass XRF testing if only the surface is analyzed. Reputable testing combines XRF with other methods.
Acid Testing (Touchstone Testing)
The oldest method. A gold sample is rubbed against a dark stone, leaving a streak. Different acid solutions are applied to the streak. The acid that dissolves the streak indicates the purity level. Stronger acids dissolve lower-purity gold while leaving higher-purity gold intact.
Acid testing is inexpensive ($20-50 for a kit), mildly destructive (removes a tiny amount of surface material), and adequate for distinguishing between major purity levels (10K vs 14K vs 18K vs 24K). It lacks the precision needed for investment-grade verification.
Additional Methods
Sigma metalytics testing: Measures electrical conductivity, which varies by alloy composition. Non-destructive and effective at detecting tungsten-core fakes, since tungsten has different conductivity than gold or gold alloys.
Specific gravity testing: Measures density by weighing the item in air and in water (Archimedes principle). Gold’s density (19.32 g/cm3) is distinctive, though tungsten (19.25 g/cm3) is close enough to require supplementary testing.
Ultrasound testing: Measures the speed of sound through the metal. Gold and tungsten have different acoustic velocities. Effective for detecting core fakes in larger bars.
Investment Gold Purity Standards
IRA Requirements
The IRS requires gold in self-directed IRAs to be .995 fine or higher (IRC Section 408(m)(3)(A)). This means:
Eligible: .999 bars, .9999 coins (Maple Leaf, Philharmonic, Kangaroo, Britannia), American Eagle (statutory exemption)
Not eligible: Krugerrand (.9167, no exemption), pre-1933 US gold coins, jewelry-grade gold
The American Eagle exception is unique. Congress explicitly authorized Eagles for IRAs despite their .9167 fineness in the Taxpayer Relief Act of 1997. No other 22K coin has this exemption.
LBMA Good Delivery Standard
London Good Delivery gold bars must be .995 fine or higher. These 400 oz bars (roughly $1 million each) are the standard for institutional trading and central bank reserves. The .995 minimum was set in 1954; most modern refining produces .999 or .9999 bars.
International Standards
Asian markets, particularly China and India, strongly prefer .999 and .9999 gold. The Shanghai Gold Exchange trades 99.99% purity as its benchmark contract. This cultural preference for high purity has driven the Canadian Maple Leaf and Australian products to strong market positions in Asia.
Why Different Coins Have Different Purities
The choice between 22K and 24K reflects different design philosophies.
The 22K Approach (Eagle, Krugerrand, Sovereign)
Adding copper (and sometimes silver) creates a harder alloy that resists scratching, denting, and edge damage. The American Eagle’s 91.67% gold, 3% silver, 5.33% copper composition produces a durable coin that handles well and shows less wear over time. The Krugerrand’s 91.67% gold, 8.33% copper mix gives it distinctive warmth and toughness.
The trade-off: lower purity means the coin must weigh more than one troy ounce in total to contain one troy ounce of pure gold. This adds a small amount of cost (the alloy metals have value, though minimal compared to gold) and creates a slightly larger, heavier coin.
The 24K Approach (Maple Leaf, Philharmonic, Kangaroo, Britannia)
Pure .9999 gold simplifies weight verification (1 oz coin weighs exactly 31.1g), meets the highest international purity standards, and appeals to markets where purity carries cultural weight.
The trade-off: pure gold is soft. A dropped .9999 coin dents. Stacked coins without protective capsules scratch each other. Storage requires more care. See our bars vs coins guide for how this affects practical ownership decisions.
Purity Comparison: Major Investment Coins
| Coin | Purity | Total Weight (1 oz) | Gold Weight | Alloy | IRA Eligible |
|---|---|---|---|---|---|
| Canadian Maple Leaf | .9999 (24K) | 31.10g | 31.10g | None | Yes |
| Austrian Philharmonic | .9999 (24K) | 31.10g | 31.10g | None | Yes |
| Australian Kangaroo | .9999 (24K) | 31.10g | 31.10g | None | Yes |
| British Britannia | .9999 (24K) | 31.10g | 31.10g | None | Yes |
| American Eagle | .9167 (22K) | 33.93g | 31.10g | Cu, Ag | Yes (exemption) |
| Krugerrand | .9167 (22K) | 33.93g | 31.10g | Cu | No |
| American Buffalo | .9999 (24K) | 31.10g | 31.10g | None | Yes |
Every coin in this table contains exactly 1.0000 troy ounce (31.1035 grams) of pure gold. The price difference between them is a function of premiums and market demand, not gold content.
Frequently Asked Questions
Is 24K gold better than 22K for investment?
Both contain the same amount of pure gold per stated weight. The 24K coins offer higher purity recognition (important in Asian markets) and simpler weight verification. The 22K coins offer durability and, in the Eagle’s case, a specific IRA advantage. “Better” depends on whether purity ranking or physical durability matters more for the intended use.
What does .9999 fine mean?
It means 9,999 parts out of 10,000 are pure gold, or 99.99% gold content. The remaining 0.01% is trace impurities that cannot be economically removed. This is the highest purity commonly produced for investment products. The Royal Canadian Mint has produced .99999 fine gold (five nines) for specialty products, but the practical difference from .9999 is negligible.
How can I tell the karat of gold at home?
A basic acid test kit ($20-50) can distinguish between major karat levels. A precision scale (0.01g accuracy, roughly $20-30) can verify weight against published specifications for known coins and bars. A strong magnet test confirms the item is not magnetic (gold is non-magnetic). These methods provide basic screening but are not definitive. For high-confidence verification, sigma metalytics testing or professional XRF analysis is recommended.
Does gold purity affect the price per ounce?
No, in standard bullion pricing. A 1 oz Eagle (.9167) and a 1 oz Maple Leaf (.9999) both contain one troy ounce of gold. Price differences between them reflect premiums, demand, and liquidity, not purity. The gold market prices pure gold content, and both coins contain the same pure gold amount.
Why is 10K the minimum to be called gold in the US?
The National Gold and Silver Stamping Act of 1906 (amended in 1961) established 10 karat as the minimum standard for articles marked as “gold” in the United States. Below 41.7% gold content, the product contains more non-gold than gold, and the designation was deemed misleading. Other countries set different minimums: the UK requires 9K (37.5%), while many European countries require 8K (33.3%).