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As far as I know, glass is a non-crystalline amorphous solid. Does glass exist in crystalline forms? Or is it the definition of glass that it's non-crystalline, in which case my question is based off of a false assumption?

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    $\begingroup$ That's right, the definition of glass is that it's amorphous, so the question as it is does not make sense. Some compounds just tend to be amorphous in certain conditions; when treated differently, they may become crystalline as well. When crystalline, however, they do not qualify as glass. $\endgroup$ – Ivan Neretin Mar 30 '16 at 16:19
  • $\begingroup$ @IvanNeretin Thank you for explaining, I'm not well versed in chemistry. $\endgroup$ – Nit Mar 30 '16 at 16:23
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    $\begingroup$ Some of the confusion may be that glass with a high lead oxide content is typically referred to a "crystal" because of its sparkle and the fact that the glass is usually cut to highlight the high refractive index. But it too is amorphous. en.wikipedia.org/wiki/Lead_glass $\endgroup$ – MaxW Mar 30 '16 at 17:49
  • $\begingroup$ Many compounds that usually form "glasses" can be cooled from the melt slow enough so they crystallise. In some cases this would take years, in others (e.g. atactic polystyrene) it is impossible. $\endgroup$ – Karl Mar 30 '16 at 21:04
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As Ivan explained, a glass is by definition amorphous. However, you can take a quartz crystal (silicon dioxide, which is the main ingredient of glass) if it is of high purity and optical quality, and machine it into a transparent object such as a window pane.

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    $\begingroup$ There are very very few quartz crystals in the world big enough to get a regular window pane. Even then it would almost certainly be flawed. $\endgroup$ – MaxW Mar 30 '16 at 19:33
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    $\begingroup$ Sapphire (i.e. corundum) is regularly grown into huge single crystals and sawn and polished into "not-glass" windows, mostly for expensive watches. $\endgroup$ – Karl Mar 30 '16 at 20:58
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    $\begingroup$ Huge crystals of quartz are also routinely produced. $\endgroup$ – Ivan Neretin Mar 30 '16 at 21:13
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Glass is by definition amorphous. If you were to crystallize it then it would be considered a ceramic, even though compositionally you have not changed your material. Often times the crystallization may occur as a result of slow cooling rates which then allows the formation of ordered arrangements of atoms (crystals) which are usually disallowed by more rapid cooling.

Sometimes partial crystallization occurs and is even desired. This produces a "glassy-ceramic" material which can give the best of both materials. Corelleware is famously strong and durable because its structure consists of ceramic particles in a glass matrix. This also gives it it's famous million needle-like shards when broken.

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