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 and glasses, both drinking and the visual aid.
On Admiral Kirk's birthday in 2285, Doctor McCoy gave him a pair of reading glasses possibly as a slight against Kirk's age. 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)