Most glass is a man-made material sometimes used to simulate a gemstone. In that case it's called paste. There are also three types of naturally occurring glass: Obsidian, Moldavite and Lybian Desert Glass.

Gemological Information for Glass
Color	All Colors
Crystal Structure	Amorphous
Refractive Index	1.47 to 1.76
Durability	Poor
Hardness	5 to 6
Similar Stones	Imitates almost any gem material
Treatments	Coatings
Country of Origin	Not Applicable
Glass Care
Ultrasonic Cleaning	Not Safe
Steam Cleaning	Not Safe
Warm Soapy Water	Safe (avoid if foil backed or glued)
Chemical Attack	Easily Attacked (even by fumes)
Light Sensitivity	None
Heat Sensitivity	Rapid heat change may cause fracture, high heat will cause it to liquify