750 Gold
The hallmark number for 18 karat gold, meaning 75% pure.
750 is 750 parts gold per 1000, the parts-per-thousand way of writing 18K.
It appears on the hallmark alongside the BIS mark and the HUID, usually on the clasp or an inner surface.
The equivalents you will meet are 999 for 24K, 916 for 22K, 750 for 18K and 585 for 14K.
18 Karat Gold
75% gold. Harder and more durable than 22K, which is why stone-set and Western-style jewellery uses it.
916 Gold
The hallmark number for 22 karat gold, meaning 91.6% pure. The same thing as 22K.
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.