Fluorescence

Fluorescence in diamonds is graded on a scale from None to Very Strong, with blue being the most common color. Strong blue fluorescence can make a slightly yellow diamond appear whiter in sunlight (which contains UV), though very strong fluorescence occasionally gives stones a hazy appearance. Rubies commonly fluoresce red under UV, which can intensify their color in daylight. The GIA grades fluorescence as part of its standard diamond grading report, and the property is widely used as a gem identification aid.