Corundum is famously known as either Ruby or Sapphire. It is an Oxide of Aluminum (Al2O3) which while naturally colorless can be a fair variety of colors based on what metal impurity it contains. If Red (Chromium), Corundum is traditionally called Ruby. If any other color, including pink, it is considered Sapphire.
Corundum is in the Trigonal crystal system and is, part of the Hematite group (M2O3 where M = Al, Fe, V, Cr, or, Ti). Corundum is found in many locales all over the world but some of the most notable specimens come from Sri Lanka and Madagascar.
What to look for: Color, Isolation and Formation of Crystals, Less or No Damage, Specimens on Matrix, Internal Clarity, Luster, Rare Localities, and, Associations.