- The Nature of Minerals
- The Formation of Crystals
- The External Form of Crystals
- Outward Appearance Of Crystalline Material
- Distorted and Deformed Crystals and Pseudomorphs
- Crystal Systems
Since crystalline materials occur in nature in several different forms, it is desirable to use a number of different terms to describe such materials, based on their appearance to the unaided eye.
CrystalsCrystals are substances that posses a crystal structure and are wholly or partly bounded by natural, plane surfaces.
Twin CrystalsSometimes two crystals, or two parts of the same crystal, are joined together along a common axis, or plane. Such a formation is called a TWIN CRYSTAL.
Crystal Aggregates, or GroupsA number of crystals sometimes grow together. Each crystal in the group is large enough to be seen easily. A group of such crystals thus grown together is known as a CRYSTAL AGGREGATE or a CRYSTAL GROUP.
Crystalline AggregatesCrystalline aggregates are composed of many very small individual crystals, often too small to be seen by the unaided eye. Metals such as gold and silver in their native state, as alloys, and even when rolled and shaped for use in jewelry, are CRYSTALLINE AGGREGATES.
Cryptocrystalline AggregatesThese are aggregates of crystals so tiny that even high magnification fails to resolve them. Their presence is proved by their effect on polarized light. Agate, carnelian and other varieties of chalcedony are examples.