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