Corundum crystals from Southern California

The mineral corundum, which is second only to diamond in terms of hardness, consists of aluminum oxide (Al2O3). Corundum comes in a variety of colors, depending on the trace amounts of other minerals (e.g., rutile = titanium oxide) it contains.

The color can be red, blue, yellow, brown, green, or purple to violet, and some crystals contain color zones. Pure corundum is white. If the color of corundum is red, it is called ruby. If the color is blue, it is called sapphire.

A friend recently gave me some corundum crystals from Cascade Canyon, San Gabriel Mountains, about 2 miles southwest of Mount Baldy, which is near the town of Upland in Los Angeles County, Southern California. 

A hand specimen (4 cm wide) containing small, scattered
 crystals of corundum. The color is between ruby and sapphire.
Most collectors would most likely refer to these crystals as ruby.

Close-up of the left-middle side of the hand specimen shown above.
The lenticular crystal in the lower right side is 4 mm long.

The corundum at the Cascade Canyon locality formed when complexly deformed sedimentary rock (of Paleozoic age) was contact metamorphosed (heated up) by small granitic intrusions (of Cretaceous age). 

If you want to see outcrop pictures and more information about this locality, just Google the phrase:  Cascade Canyon ruby