Corundum is the mineral that produces the blue variety called sapphire.
Corundum is crystalline aluminum oxide with the formula Al₂O₃. Pure corundum is colorless, but trace elements create its gem colors. Chromium produces the red variety ruby, while iron and titanium commonly contribute to the blue color of sapphire.
Sapphire is the gem name for gem-quality corundum of colors other than red, although blue is its best-known color. Corundum forms in aluminum-rich, silica-poor rocks and can survive weathering, allowing crystals to accumulate in alluvial deposits. Synthetic corundum is also manufactured for abrasives, watch crystals, bearings, and laboratory applications.
Corundum is extremely hard and ranks 9 on the Mohs scale, but hardness is not the same as resistance to breaking. Diamond is harder, while some tougher materials may resist impact better.