Interesting story and research at http://phys.org/print358750569.html … molecular scientists unexpectedly produced a new type of glass, and it involved vapourising the experimental material – thereby imitating lightning or meteors and the creation of fulgarites. The study, published in PNAS (August 2015), and in Materials Science journal, produced a glass where the molecules were aligned in an orderly fashion in a single direction, which is a novelty (and would, presumably, give glass greater strength). It occurred at a certain temperature – and not at lower or higher heat levels. The point is that glass is a halfway house between a semi liquid and a solid. As a result of its origin in runny fluid like material the molecules tend to be chaotic, going in different directions. Glass shatters and so does flint which thereby limits what you can do with them, although the fact that flint shatters is one of the reasons it is easy to work and make cutting tools, scrapers, and points.