A glass, as I understand it, is a material whose solid phase is amorphous and which undergoes a glass transition when heated.
My train of thought - and therefore my questions - began with obsidian. My understanding is that obsidian is formed from material that would normally form crystalline rock, but since it's exposed to air during an eruption, it cools too rapidly to crystallize and therefore solidifies as glass. This led me to wonder: how large can a lava stream be and still produce obsidian? In particular, in season 7 of Game of Thrones, the island of Dragonstone is revealed to contain a veritable mountain of obsidian - is such a thing possible in the real world? I would have thought that, above a certain volume, the center of the mass would be well-enough insulated so that crystallization could occur, spoiling the obsidian. What's the largest mass of obsidian found in nature?
Thinking about too-rapid cooling led me to another question: can water form glass? Is it possible to blast-freeze water so quickly that ice crystals do not form? If so, what would be the properties of such a glass?
Finally, I wonder about regular, garden-variety silicate glass (as found in windows and bottles.) Can it be cooled slowly enough so that crystals do form? And if so, what are its properties? (Have I essentially re-invented quartz?)