Most copper bullion carries the same three characters somewhere on its face: .999. It's a common marking and one that rarely gets explained. Here's what it actually tells you, what it doesn't, and how to sanity-check a piece already sitting in your drawer.
Fineness is a percentage, written differently
Fineness expresses purity in parts per thousand rather than parts per hundred. So .999 fine means 999 parts copper out of 1,000, or 99.9% copper by mass. The remaining 0.1% is trace material — usually oxygen, sulphur, silver or iron left over from refining. It is not an additive or an alloy; it's residue.
You'll see the same convention across the metals: .999 silver, .9999 gold ("four nines"), .9995 platinum. Each extra nine is a tenfold reduction in what's left over, which is why refiners talk about "three nines" and "four nines" copper rather than reciting decimals.
One thing fineness does not tell you is weight, dimensions, or who made the piece. A stamp reading ".999 FINE COPPER 1 TROY OZ" is making two separate claims — a purity claim and a weight claim — and they should be checked separately.
How .999 compares with industrial copper
Copper is unusual among bullion metals because the industrial world already refines large quantities of it to a similar standard. The commonly specified grades sit in roughly this band:
- C11000, electrolytic tough pitch (ETP) — typically specified at a minimum of 99.90% copper. This is the workhorse grade used for building wire and busbar.
- C10200, oxygen-free (Cu-OF) — typically a minimum of 99.95%, with the oxygen content deliberately kept low.
- C10100, oxygen-free electronic (Cu-OFE) — typically a minimum of 99.99%, used where conductivity and hydrogen embrittlement matter.
So .999 fine bullion copper sits in the same range as standard high-conductivity industrial copper. That matters for two reasons. First, the purity claim on a copper round is unremarkable metallurgically — the feedstock is a normal commercial product, not something exotic. Second, a piece marked .999 should behave like high-conductivity copper does: the right density, the right colour, non-magnetic, and prone to toning over time.
It also shapes what you are paying for. Copper is a low-value metal by weight compared with gold and silver, so on a one-ounce piece the striking, finishing and packaging make up a much larger share of the price than the metal itself.
Troy ounces, grams, and why your kitchen scale lies
1 troy ounce = 31.1035 grams. The everyday avoirdupois ounce used for food is 28.3495 grams. A troy ounce is about 10% heavier, so a 1 oz copper piece that weighs "an ounce" on a kitchen scale set to imperial is actually underweight.
The numbers worth memorising:
- 1 troy oz = 31.1035 g
- 5 troy oz = 155.52 g — the weight of a 5 oz bar
- 1 kilogram = 32.1507 troy oz
- A tube of 20 rounds = 622.07 g of copper, before the tube itself
Mints work to a tolerance, so a genuine piece will sit slightly either side of nominal. A gram or more light on a 1 oz piece is a reason to look harder.
Five checks you can do at home
1. Weigh it properly
A jeweller's scale reading to 0.01 g is inexpensive and settles most questions. Set it to grams, not ounces, and compare against the figures above.
2. Measure it and work out the density
Copper has a density of about 8.96 g/cm³. A 1 oz piece of pure copper therefore occupies roughly 3.47 cm³. Measure diameter and thickness with callipers, calculate the volume, and divide the mass by it. Copper-plated zinc (about 7.1 g/cm³) or plated steel (about 7.9) will come out noticeably light for its size; lead-cored fakes come out heavy. Brass sits close enough to copper that density alone won't separate them — colour and a look at the edge will.
3. Try a magnet
Copper is not magnetic. Nothing should stick. On a thicker piece, a strong neodymium magnet slid across the surface may drag slightly as eddy currents form — that's normal and is a good sign, not a bad one. A piece that a magnet actually grips is plated steel.
4. Listen, but don't rely on it
Copper gives a shorter, duller tone than silver. The ring test is genuinely useful on silver and close to useless as a standalone test on copper. Treat an odd, dead thud as a prompt to weigh and measure, not as a verdict.
5. Look at the edges and high points
Plating fails first where the metal is thinnest and most handled — the rim, the raised design elements, the corners of a bar. A different colour showing through, or a hard line where a coating ends, is the clearest visual tell. Even toning from bright orange through to deep brown, on the other hand, is just oxidation. Every copper piece does it, and it doesn't affect purity or weight.
Markings beyond the fineness stamp
Alongside purity and weight, a piece may carry a maker's mark, a design or series name, a year, or occasionally a batch number. None of that changes the metal. Those marks are identifiers, useful mainly for matching a piece to how it was described when sold, and for noticing an obvious mismatch — a font or design element that doesn't fit the rest of the piece.
Treat markings as supporting evidence rather than proof. A stamp is the easiest part of a piece to copy; mass, volume and magnetic behaviour are not. If you bought from Global Copper and something doesn't look right, email sales@globalcopperbullion.com or use the contact page, with a clear photo of both faces and the edge.
A note on GST
In New Zealand, GST-exempt "fine metal" treatment applies to investment gold, silver and platinum at set purities. Copper is not on that list, so copper bullion is sold GST-inclusive. Every price on the shop — from a 1 oz bar upward — is quoted in NZD with GST already included, rather than quoted GST-exclusive the way exempt metals can be. That is simply how the rules are drawn for copper, and your own tax position is worth confirming with your accountant.
The short version
.999 means 99.9% copper, which puts bullion copper in the same purity band as standard high-conductivity industrial grades. The weight stamp is in troy ounces at 31.1035 g each, not the ounce on your kitchen scale. And a set of callipers, a 0.01 g scale and a magnet will answer more questions in two minutes than any amount of squinting at a stamp.