Crystals as geometrical forms
Crystals are now defined as bodies which have definite geometrical forms bounded by flat faces and straight edges. The crystalline state, of course, is not only found among natural minerals, but also occurs in many artificially produced substances, such as refined salt and sugar, and metals, and among waste products of living organisms. The crystalline form is really the external expression of the internal atomic structure of the mineral. The constituent atoms of any given mineral species are arranged in a definite geometric pattern known as the crystal lattice. This lattice determines the mineral’s physical properties, which include hardness, specific gravity, the ease and direction of splitting, and the effects on light transmitted through it .Though the internal structure of any given mineral is always the same, the shape of the crystal is often imperfect, since the growth of crystals in nature is always dependent on the space available to them. The name crystal, however, is used not only for the perfect form, but also for any part of a crystalline substance which has at least some crystal faces and edges. Not all minerals are crystalline. Some are completely structureless and are termed amorphous, but these are relatively rare among natural minerals, the best known being opal and amber. Amorphous minerals are never bounded by flat surfaces, and never have an ordered lattice structure.
They usually have roundish, indefinite shapes (Plate 15). Under certain conditions, an amorphous mineral may, in the course of time, become finely crystalline. Opal, for instance, tends in due course to change into the minutely crystalline {cryptocrystalline} mineral chalcedony.