# Glass
For worthless pieces of colored glass, see Gem.
Glass is a material that appears in NetHack. Most glass items are fragile and break when thrown or subjected to certain kinds of blunt force. However, glass artifacts and gems are not fragile.[1]
"Crystal" objects in NetHack, excluding the dilithium crystal, are actually made of glass.

Items made of glass
Crystal plate mail
Mirror
Crystal ball
Pair of lenses
All potions
Wands with the randomized appearance of glass wand or crystal wand
All colors of worthless glass
References

↑ breaktest in dothrow.c gives all artifacts a 99% chance of resisting, then ensures that glass artifacts do not break.