The External Form of Crystals




A regular internal structure means that if growth is not confined by other solids, definite geometrical forms bounded by plane (flat) surfaces, FACES will result. These crystal forms reflect the internal arrangement of the particles that comprise the material. Crystalline material that has assumed a definite geometric form is known as a CRYSTAL. Such crystals may vary from scores of feet in length to very small particles. They may be so small that in crystalline aggregates they can only be recognized as individual crystals by a powerful microscope. Beryl crystals have been known to weigh more than a ton, and spondumene crystals have been found that weighed as much as 37 tons and measured 47 feet in length. These, of course, were not of gem quality.

Planes of Atoms and Crystal Faces

All crystalline substances grow or enlarge gradually by the orderly deposition of layers or PLANES, of atoms, the distances between which are of course, different in various substances. It is easier to understand crystal growth and crystal forms by thinking of the crystal faces as corresponding to such planes of atoms with the atoms in each layer arranged in a definite pattern. Each mineral species has a characteristic pattern of its own that results in characteristic external forms, or crystal shapes. Sometimes material with the same chemical composition may assume different atomic arrangements or patterns. In this event, each is a separate and distinct mineral. For example DIAMOND AND GRAPHITE are both wholly composed of CARBON, yet their properties are amazingly different because of the differences in crystal structure.

Crystal or Crystalline Material

Often a crystalline structure does not result in a definite geometric external form that is recognizable to the eye. Such material, though having an internal crystalline structure appears to the unaided eyes as a more or less shapeless mass and is known as crystalline material with a MASSIVE form. This is most commonly encountered when the position in which the mineral grew was confined by other growing crystals.

Glass

It is obvious that, scientifically, the term crystal means WITH orderly internal structure, whereas the term glass means WITHOUT orderly internal structure. It is correct to refer to an amethyst necklace as being crystal, but a manufactured glass imitation of amethyst is NOT crystal. Also, a manufactured glass replica of a rough diamond crystal is not crystal. It is clear from this that fine lead glassware is incorrectly called "crystal." Certain naturally occurring substances possessing neither orderly arrangement of atoms nor constant chemical formulae, such as obsidian, are known as NATURAL GLASSES.