Gemstone buying guides
Each guide covers what to look for, which treatments must be disclosed and which certificates carry weight — alongside the verified dealers admitted for that stone.
Alexandrite is the colour-change variety of chrysoberyl: green to bluish-green in daylight, red to purplish-red under incandescent light.
Aquamarine is the blue to greenish-blue variety of beryl, the same mineral family as emerald and morganite.
Demantoid is the green variety of andradite garnet.
Emerald is the green variety of beryl, and chromium and vanadium give it the colour.
Kunzite is the pink to violet variety of spodumene, named for the gemmologist George Frederick Kunz who described it in 1902.
Moonstone is the best known of the phenomenal feldspars, defined by adularescence, the soft billow of light that moves across the stone as it turns.
Morganite is the pink to peach member of the beryl family, sharing its species with emerald and aquamarine.
Opal is valued for play-of-colour, the flashes of spectral colour that move across the stone as it turns.
Peridot is among the small number of gems that occur in one colour only.
Ruby is red corundum, and chromium gives it the colour.
Sapphire is the blue variety of corundum.
Spessartite is the manganese garnet, prized for an orange that no other gem family delivers with the same conviction.
Spinel is a magnesium aluminium oxide.
Tanzanite is the blue to violet variety of zoisite.
Topaz is two markets under one name.
Tourmaline comes in more colours than any other gem family.
Tsavorite is the green grossular garnet found along the border of Kenya and Tanzania, coloured by vanadium and chromium.
Zircon is the oldest mineral ever dated on earth and one of the most misunderstood gems in the trade, routinely confused with the synthetic cubic zirconia despite having nothing in common with it.