Glass was a generic term for any molten silicon which could be naturally occurring (volcanic) or man-made and was, by definition, very fragile. Glass was most commonly used in windows, corrective lenses, and drinking glasses.
On Admiral Kirk's birthday in 2285, Doctor McCoy gave him a pair of reading glasses. The glasses later were broken in a battle with the USS Reliant commandeered by Khan Noonien Singh. The glasses were sold in 20th century Earth at an antiques dealer in San Francisco who purchased them for US$100, noting they would have been worth more if the lenses had been intact. (Star Trek II: The Wrath of Khan; Star Trek IV: The Voyage Home)
An invidium contamination aboard the USS Enterprise-D caused a drinking glass to melt in the hand of its holder, Lieutenant junior grade Duffy releasing the beverage from within. (TNG: "Hollow Pursuits")
After she was beamed aboard the USS Enterprise-E, Lily Sloane was surprised to see no glass covered a hatch to space. Jean-Luc Picard then demonstrated the function of a force field to her. (Star Trek: First Contact)