22k, 18k, or 14k? Why Gold Purity Changes Everything About Melt Value
Ask someone how much a "10-gram gold coin" is worth and you've actually asked an unanswerable question. Ten grams of what purity? The gap between a 24-karat coin and a 14-karat one at identical weight isn't a rounding error — it's a difference of hundreds of dollars, and it's the single most common thing people get wrong when eyeballing gold value.
What karat actually measures
Karat measures gold purity out of 24 parts. 24k is (nominally) pure gold — in practice usually .999 or .9999 fine. 22k is 22/24 = .9167 fine, the traditional purity for many gold bullion and circulating gold coins, including the American Gold Eagle and the historic British sovereign. 18k is .750 fine — 75% gold, 25% other metals (usually copper and silver) added for hardness and color. 14k, common in American jewelry and some older gold coinage, is .5833 fine — barely over half gold by weight.
The rest of the alloy in anything below 24k isn't worthless, but it isn't gold either, and spot price only pays for the gold. This is exactly why melt-value math always starts with weight times purity, never weight alone.
Reading purity marks you'll actually see
Karat notation ("22k," "14k") is common in the U.S. and on many coins, but jewelry and coinage worldwide also use decimal fineness marks stamped directly on the metal: .999 or .9999 for investment-grade fine gold, .916 or .9167 (equivalent to 22k), .750 (18k), .585 or .583 (14k), and .375 (9k, common in older British jewelry but rare in coinage). Some historic mints used their own hallmark symbols instead of a printed number, which requires a reference guide to translate. Whenever a purity mark is present in any format, it's more reliable than a visual guess -- gold's color alone is a poor purity indicator, since the choice of alloying metal (more silver versus more copper) changes a coin's color almost as much as its actual gold content does.
The same 10 grams, three different purities
Take a hypothetical 10-gram coin at a gold spot price of $2,400/troy oz and run it through the Coin Melt Value Calculator at three different purities:
- 24k (.999 fine): 10g × 0.999 = 9.99g pure gold → 0.3212 troy oz → ≈ $770.85 in melt value.
- 22k (.9167 fine): 10g × 0.9167 = 9.17g pure gold → 0.2947 troy oz → ≈ $707.34 in melt value.
- 14k (.5833 fine): 10g × 0.5833 = 5.83g pure gold → 0.1875 troy oz → ≈ $450.08 in melt value.
Same weight on a scale. A $320 gap between the 24k and 14k figures — purely from purity, before anyone has looked at the coin's condition or rarity. If you only weigh a coin and skip the purity question, you can be off by 40% or more.
Frequently asked questions
Is 24k gold ever actually 100% pure? No -- even coins marketed as ".9999 fine" ("four nines") contain a trace of other elements; true 100.000% pure gold isn't practically achievable at scale. For melt-value purposes, .999 or .9999 is close enough to treat as fully pure.
Why do coins use less than 24k gold at all, if buyers want purity? Pure gold is very soft -- a coin struck from .9999 fine gold wears and scratches far more easily than an alloyed coin. Historic circulating gold coinage almost always used an alloy specifically for durability.
Can I estimate purity without any stamp at all? Specific-gravity testing and XRF scanning both work without a stamp, but both need equipment most collectors don't own -- for anything valuable and unmarked, a jeweler or coin dealer with proper testing tools is the reliable path.
Purity standards vary by country and era
Karat and fineness aren't universal constants chosen once and used everywhere -- different countries and different eras settled on different standard purities for their circulating gold coinage, and knowing a coin's country and period of origin often tells you its purity before you even check a stamp. The historic British sovereign was struck at .9167 fine (22k) for most of its production history. Pre-1933 United States gold coinage was struck at .900 fine -- a purity unique enough to U.S. coinage of that era that it's often simply called "coin gold" in the trade, distinct from both 22k and 24k standards used elsewhere. French and Swiss 20-franc gold coins were also struck at .900 fine, matching the U.S. standard closely despite being entirely separate currencies -- a reminder that international gold coinage in the 19th and early 20th centuries often converged on similar purities simply because the practical tradeoffs (durability versus gold content) were the same everywhere.
Where purity is stamped — and where it isn't
Modern bullion coins almost always state their purity directly: an American Gold Eagle is marked "22K" on its edge design (it's actually .9167 fine, alloyed with silver and copper for durability, though it's sold and priced as if it contained a full troy ounce of pure gold — the coin's total weight is higher than 1 oz specifically to compensate). A Canadian Gold Maple Leaf is stamped .9999 fine. Older and foreign gold coinage is far less consistent: pre-1933 U.S. gold coins are .900 fine, British sovereigns are .9167 fine, and plenty of historic gold coins from other mints fall at purities in between, established by that country's own coinage laws at the time.
When purity isn't marked or known, a jeweler's acid test, an XRF (X-ray fluorescence) reading, or specific-gravity testing can identify it — but guessing is how people either badly overpay for "gold" jewelry that's mostly alloy, or badly underprice a genuinely high-purity coin.
Purity vs. numismatic value — two separate questions
None of this purity math says anything about whether a specific gold coin is worth more than its melt value as a collectible. A common-date American Gold Eagle in circulated condition trades close to its metal content; a rare date, a low-mintage year, or a coin in exceptional condition can carry a real premium regardless of purity. Purity tells you the floor. Rarity and grade tell you whether the coin clears that floor by a little or a lot — which is exactly what the Mintage-Rarity Estimator and Coin Grading Scale Guide are for.
The takeaway
Before valuing any gold item by weight alone, find the purity mark, verify it if there's any doubt, and only then run the weight-times-purity math. A scale tells you what something weighs. Only purity tells you what fraction of that weight is actually gold — and that's the number the spot price is paying for.