Perhaps you think of a crystal as a rare and beautiful mineral or gem. You are partly right. Emeralds and diamonds are crystals. But not an crystals are rare and beautiful. Each tiny particle of salt and sugar is also a crystal! Many of the common substances around us are crystals.

A crystal is the solid form of a substance. It has a definite shape and a definite number of faces because of the arrangement of its atoms. All crystals of the same substance have the same shape, though they may differ in size.

In nature, there are hundreds of substances which form crystals Water is one of the most common. When water freezes, it turns into frost crystals or snowflakes.

Mineral crystals are also formed in certain rock-making processes. Great quantities of hot and melted rock material deep down in the earth are actually solutions of minerals. When these masses of molten, or melted, rock materials are forced upward into the earth’s crust, they begin to cool.

They cool very slowly. The minerals become crystals when they change from the hot liquid to the cool solid form. For example, the rock granite contains crystals of the minerals quartz, feldspar, and mica. Many millions of years ago, this granite was a molten mass of minerals in solution. In the same way, there are today within the earth’s crust masses of molten rock material which are slowly cooling and forming mineral crystals of many kinds.

Crystals take many shapes. All the crystals known in the world can be grouped into 32 forms, which are in turn grouped into six systems. Crystals can also be of many sizes. Some minerals form crystals that can be seen only through a microscope. Others form crystals that weigh several hundred pounds each.