Topaz
A hard silicate gem, most often seen as blue topaz. Frequently confused with, and substituted for, yellow sapphire.
Natural topaz is usually colourless, pale yellow or brown. The vivid blues that dominate the market are irradiated and heated.
It is 8 on the Mohs scale, hard but with perfect cleavage in one direction, so a sharp knock can split it cleanly.
Yellow topaz and pukhraj are not the same stone. Pukhraj is yellow sapphire, which is corundum and considerably harder.
When you are buying
Because of the cleavage, topaz is a poor choice for a ring worn daily. It is a much safer stone in earrings and pendants.
Yellow Sapphire
Yellow corundum. Pukhraj in Hindi, the stone of Jupiter, and the most commonly prescribed astrological gem in India.
Citrine
The yellow to orange variety of quartz. Most of it is heat-treated amethyst, and it is often mis-sold as yellow sapphire.
Aquamarine
The pale blue variety of beryl, the same mineral family as emerald. Usually clean and untroubled.
Gemstone
Any mineral or organic material cut for jewellery. Traditionally split into precious (diamond, ruby, sapphire, emerald) and semi-precious.