London is the world's largest physical gold market and the US is the worlds largest precious metals futures market. Remarkably both still use a sixteenth century unit of weight - the Troy Ounce, detailed below - and this is the unit global media use when reporting precious metal prices
What are Troy ounces?
One troy ounce = 31.1035 grams. 32.1507 troy ounces = one kilogram.
Why are Troy ounces used for precious metals in the UK and the USA ?
Troy weights predate the metric system by centuries - the, now pretty much obsolete, pennyweight is the weight of one Elizabeth I sterling silver penny. Twenty of these silver pennies weigh one troy ounce and twelve troy ounces make one troy pound.
Predecimalisation in the UK 12 x 20 currency (old) pennies equaled one pound sterling so common denominators facilitated calculations of value along with a simple weight check system for precious metals.
It is also why the UK currency is still known as sterling.
What are gold carats
Gold carats are completely different to diamond carats which are a measurement of a diamond's weight. Gold carats are a measurement of a metal item's purity in terms of the gold content. 24 carat gold is pure gold and so a 9 carat gold item contains 9/24th gold, 18 carat gold contains 18/24 gold and so on.
9 carat gold jewellery is sometime marked 375 indicating a gold content of 375 parts per 1000 gold (9/24= 375/1000), similarly 18 carat gold often being marked 750.
Platinum and palladium jewellery items are usually hallmarked parts per thousand.
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Handy weight-checks with new UK coins currently 20p coin = 5.0grams, 50p coin= 8.0 grams, £2 coin = 12.0 grams
Precious Metal Prices & London Fixes
Gold, platinum and palladium are global markets operating nearly 24 hours each weekday. Australia and FarEast trading overlaps with Europe and London which in turn overlaps with the USA a market break that varies with local seasonal clock adjustment is usually around 22.15 hours GMT
Because market prices can change continuously a system known as the London gold fix, paralleled with 'fixes' for palladium and platinum, evolved. Twice each weekday a group with instructions to buy and sell good delivery refined metals on behalf of central banks, commercial banks, users and traders fix the price at that moment in time and that fixed price becomes a matter of record reported globally. Thus buyers and sellers of precious metals know exactly the metal prices applicable in their transactions.
In 2015 the fix system was reset so that it could continue to function but with greater transparency to regulators.