Karat
The purity of gold, measured in twenty-fourths. 24K is pure gold; 22K is 22 parts gold to 2 parts other metal.
Pure gold is defined as 24 karat. Every lower number tells you how many of 24 parts are actually gold, so 22K is 22/24, or 91.6%, and 18K is 18/24, or 75%.
The rest of the alloy is copper, silver, zinc or nickel, added because pure gold is too soft to hold a stone or survive daily wear.
Karat says nothing about weight, colour or workmanship. A heavy 18K piece contains far more gold than a light 22K one.
What you will hear
“Higher karat always means a better piece of jewellery.”
In fact: Higher karat means softer metal. For a ring, a stone setting or anything worn every day, 18K outlasts 22K and holds its shape better.
When you are buying
Compare pieces on gold weight multiplied by purity, not on the karat number alone. A seller quoting only karat is quoting only one of the two numbers that set the price.
Carat
A unit of gemstone weight, equal to 0.2 grams. Nothing to do with karat, which measures gold purity.
22 Karat Gold
91.6% gold. The Indian standard for traditional jewellery, and what a 916 hallmark means.
18 Karat Gold
75% gold. Harder and more durable than 22K, which is why stone-set and Western-style jewellery uses it.
24 Karat Gold
Pure gold, 99.9% fine. Too soft for jewellery that gets worn, so it is bought as coins and bars.
Hallmark
An official stamp certifying precious metal purity. In India, BIS hallmarking is mandatory for gold jewellery.
Fineness
Purity expressed in parts per thousand rather than karats. 999, 916, 750 and 585 are the numbers you will see stamped.