Ruby: red corundum, inside other rocks
The gem ruby in a jeweller's window and the ruby in a bowl of tumbled stones are the same mineral. What changes is the rock around it.
Ruby is corundum, crystallised aluminium oxide. Pure corundum is colourless. A trace of chromium turns it red, and the same mineral in any other colour is sold as sapphire. At 9 on the Mohs scale it is one of the hardest natural minerals, which is why tiny ruby crystals survive in rock long after softer minerals have worn away.
Clear, gem-quality ruby is rare and is cut and sold by the carat. Most ruby in the crystal trade is mineral-grade. It is opaque or cloudy, full of inclusions and cracks, and is cut into beads, tumbled stones and carvings. It is still real ruby, just not gem ruby, and it should be priced and labelled that way.
Much of that mineral-grade ruby is sold still sitting in the rock it grew in. Ruby zoisite pairs red ruby with green zoisite and black flecks of hornblende. Ruby kyanite sets ruby in blue, bladed kyanite. Ruby in fuchsite puts it in sparkly green mica. Each rock gives a different colour pair, and that contrast is what customers notice from across the shop.
Ruby is the July birthstone and the traditional gift for a 40th wedding anniversary. For a shop, ruby in zoisite or kyanite is the affordable way to sell that story. The colours are bold, the stones are hard enough to tumble and carve, and the name does a lot of the selling.
Same mineral as the gem in the jeweller's window. The difference is the rock around it.
