What is a crystal?
Crystallisation can occur in solids where the constituent atoms, ions, or molecules are arranged into well-defined three-dimensional structures. When crystals are composed of ions, they tend to form giant ionic lattices, for example sodium chloride (table salt). One of the most abundant crystal minerals is quartz, with the chemical formula SiO2 (this means it has one atom of silicon bonded to two atoms of oxygen, silicon dioxide). This chemical makes up the vast majority of crystalline substances found in the ground, amethyst, agates and geodes all contain silica.
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