Sapphires are the second hardest crystal next to diamonds. 

They come in blue, violet, green, yellow, orange, pink, purple, and intermediate hues. Some stones exhibit the phenomenon known as color change, most often going from blue in daylight or fluorescent lighting to purple under incandescent light. 

Sapphires are commonly treated to improve both the color and clarity of the gemstone. Untreated clean sapphires are very rare and are sought after by collectors.