Definition: Solid substance whose structure is essentially symmetrical and geometrical. * Crystals are vital to photography as silver halide in light-sensitive emulsions. * Crystals are also vital to computers as they are used in the clocks that regulate and time every computer operation. * Most natural crystals e.g. calcite are birefringent or doubly refracting.
Glass not crystal
Glass, familiar to us as the key component of camera lenses, mirrors, etc. is not a crystal despite appearances. Technically it is a solidified liquid: when sufficiently heated up, it simply loses its shape - but not its optical properties.

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