It's hardness of 8, and lack of cleavage makes for a tough and durable gemstone. It mimics corundum, and crystals are found in a wide spectrum of colors, including red, orange, yellow, dark green, blue, purple, brown and black. Preciosa offer an excellent black colour which is supported with ideal characteristic for wax casting. Heat resistance of this material is up to 1100°C/2012°F