Corundum

Very hard (H=9) aluminum oxide (Al₂O₃) mineral with barrel-shaped, hexagonal (6-sided) crystal form; known as ruby (red & pink) when containing impurity chromium; sapphire (blue) when containing iron & titanium impurities; yellow when containing trace of iron.

Main Photo Credit: "Corundum" by NRCan, 2026. Used with permission.