Corundum

Corundum is a naturally occurring mineral comprised primarily of aluminum oxide (Al₂O₃). It is known for its hardness, ranking 9 on the Mohs scale, making it one of the hardest substances in nature. Corundum can take on various colors depending on the presence of trace elements, resulting in well-known varieties such as: 1. **Sapphire**: Typically blue but can come in many other colors except red.