Mineral composed of magnesium aluminum oxide (MgAlO). Also called magnesia spinel, its colour, due to various impurities, ranges from blood-red to blue, green, brown, and colourless. Spinel is found in basic igneous rocks, granite pegmatites, and contact metamorphic limestone deposits, often in association with corundum. Synthetic spinel has been manufactured since the early 20th century for use as imitation gem stones. Spinel may also refer more broadly to any of various mineral oxides of magnesium, iron, zinc, or manganese in combination with aluminum, chromium, or iron.