Most crystals are found in rocks that have a volcanic origin. As the molten rock cools, different substances will crystallize.

It is the rate of cooling that determines the size and type of the crystals that are formed. The slower the cooling process the larger the crystal formed. Some can grow to immense sizes, such as one particular Quartz crystal found in Kazakhstan, which was recorded to have weighed in at 77 tons.

What defines a crystal is its well ordered internal structure, which is reflected in its outward growth, characterized by smooth plane surfaces and regular geometrical shapes.

Crystals can also form when water evaporates leaving behind mineral deposits. Again the speed of evaporation can determine the size of the crystals formed. Geodes, which are hollow or partially hollow, can contain well-formed crystals as a result of minerals being deposited by water.

Amethyst Geode

Fluorite Crystals