Minerals

The faces and angles of natural crystals result from the orderly arrangements of the atoms and molecules that make up a crystal. The relation between crystal shape and microscopic structure was suggested in the seventeenth century by Robert Hooke and Christian Huygens. It was confirmed in the twentieth century with the development of x-ray diffraction, a technique that uses x rays to examine the atomic structures of materials.

In modern terms, a solid substance is considered to be crystalline if its atoms or molecules are arranged in an orderly pattern that repeats at regular intervals. Therefore metals are crystalline, although the individual crystals making up a lump of gold are too small to see with the naked eye. By contrast, atoms making up glass do not have any orderly atomic arrangement. Therefore, glass, even it if is carved into the shape of a crystal, is not a crystalline material. Natural glasses such as obsidian (volcanic glass) are not technically minerals. Non-crystalline solids are called amorphous (without form).

Although there are thousands of different minerals, the shapes of their crystals can be described using just six basic geometric forms. These are called crystal systems. To determine what crystal system a mineral belongs to, it is nesessary to obtain a well-formed specimen, then observe the number and shape of the faces and the angles at which they meet. This task may be complicated by the fact that each crystal system includes several different forms, and a single crystal may combine several forms in its shape.

For example, consider the isometric system. This is the most symmetrical system, meaning that it has the greatest amount of "sameness" in its faces and angles. In fact, the basic geometric shape of the isometric system is a cube, having all sides of equal length and with all angles equal to 90°. Halite crystals, which are cubic, are easily recognized as belonging to the isometric system. However, 15 forms are possible within the isometric system. Isometric mineral crystals include the octahedral (eight-sided) spinels, and the dodecahedral (12-sided) garnets. A single crystal combining several forms can look almost spherical.