Corundum

Corundum from the Zazafotsy Quarry, Zazafotsy, Madagascar, measuring 82 × 89 × 56 mm and weighing 774 grams. By far the heaviest specimen in this sale, a substantial chunk of one of the hardest common minerals there is.

Corundum is aluminium oxide, Al2O3, second only to diamond on the Mohs hardness scale, and its gem varieties, ruby and sapphire, are familiar to almost everyone even though most Corundum is far too included or poorly coloured to cut. Madagascar's Corundum occurs in metamorphic and metasomatic rocks associated with the island's ancient Precambrian basement, where the right combination of aluminium-rich rock and the right pressure and temperature conditions allowed coarse crystals to grow. Zazafotsy, in the central highlands, is known for producing Corundum in this kind of rough, massive habit rather than the gem-clear crystals the same province can also yield.

Madagascar has become one of the world's major sources of Corundum over the past few decades, both for gem rough and for mineral specimens showing the species in its natural matrix and habit. A piece of this size, from the David Vince Collection, is a good way to appreciate just how substantial Corundum crystals can grow before colour and clarity ever become part of the conversation.