Gemstones are measured in carats, abbreviated “ct” (and not to be confused with karats, with a “k”, used for gold purity).
A carat is one fifth of a gram – or 200 milligrams.
A point is one hundredth of a carat – or 2 milligrams.
As an indication, a one carat round diamond would be about 6.5mm in diameter.
Different stones have different densities. For example, ruby and sapphire are more dense than diamond. Thus, a one carat ruby, or a one carat sapphire, is smaller than a one carat diamond.
In the United States and some other countries, the purity of gold is expressed in karats. The karat number refers to the parts of pure gold per 24 in the alloy. Thus, 12K gold has 12 parts gold per 24, or 50%, while 24K gold is pure old.
Many European countries mark gold with a three-digit number indicating the parts per thousand of gold in the alloy. Thus, an item marked “750” is 750/1000 pure gold – or 18K.
“Gold filling” is the practice of mechanically applying a thin sheet of gold to the surface of a piece that is made of some other metal. Gold filling is much thicker than gold plating in which a layer of gold only a few microns thick is electrically deposited on the surface of the piece. Gold filled pieces are marked with the fraction of gold by weight in the piece. For example, an item marked “1/10 18K GF” would have a layer of 18K gold (75% pure) making up one-tenth of the weight of the item.
Did You Know?
A pound of feathers weighs more than a pound of gold!
(because feathers are measured in the avoirdupois system of 16 ounces to the pound whereas gold is measured in the troy system of 12 ounces to the pound.)
Silver, and sometimes gold, is measured in Troy ounces. A Troy ounce weighs about 10% more than the avoirdupois ounce that is commonly used for other items. A troy ounce is divided into pennyweight which, in turn, are divided into grains. One troy ounce equals 20 pennyweight; one pennyweight equals 24 grains.
To make matters even more confusing, one pound troy equals 12 troy ounces (not 16).