Final answer:
Gem corundum comes in various colors and types depending on trace elements and crystal lattice variations. Rubies are red due to Cr³⁺ replacing Al³⁺ in the lattice; fancy sapphires come in multiple hues based on different trace elements, and asterism is caused by inclusions in corundum.
Step-by-step explanation:
Gem corundum is a crystalline form of alumina (Al2O3) and can vary in color depending on trace elements present in its structure. When a small amount of Al³⁺ ions are replaced by Cr³⁺ ions in the corundum structure, the mineral turns a deep red and is known as ruby. A pure corundum crystal is colorless, but when trace elements like iron, titanium, or chromium are present, it can appear in a variety of colors and is referred to as fancy sapphire. Corundum can also show a phenomenon called asterism, where inclusions cause the appearance of a star-like reflection on the surface of the stone. The colors of gem-quality corundum varieties are primarily due to the size and distance of the Cr-O bonds within the crystal lattice, which affects the wavelengths of light absorbed and transmitted, thereby giving each variety its distinct color.