All gemstones can be classified as either:
- No orderly internal atomic structure.
- No naturally-occurring characteristic shape.
- Products of rapid cooling.
- Physical properties are constant in all directions.
- Definite & regular internal atomic structure.
- Geometrical external forms.
- Directional properties.
- Products of slow cooling.
- Identical in all crystals of a given species.
Gemstones can also be:
A term used to describe a crystalline substance that does not have a naturally-occurring characteristic shape. (i.e. Rose Quartz)
This type of material has not grown as a single crystal but as an aggregate of small or microscopic crystals randomly orientated within the body of the gem.
These gems are usually semi-translucent or opaque and slightly porous. Although all crypto-crystalline gems are massive, it should be noted that not all massive materials are crypto-crystalline. (i.e. Chalcedony Quartz)
The characteristic crystal form displayed by a mineral.
Crystallography is defined as the study of crystals and their structure.
There are 3 elements of symmetry that are fundamental to the study of crystallography.
Symmetry: correspondence, the similarity in or between the opposing sides of an object, considered as halves on either side of a central line, in form and dimensions.
Axis of Symmetry: an imaginary line passing through a crystal that will permit a characteristic representation of a crystal 2, 3, 4 or 6 times during each 360-degree rotation.
The plane of Symmetry: this is a plane through a crystal which divides the crystal into two mirror-image halves.
Center of Symmetry: a crystal possesses a center of symmetry when identical faces and edges occur on exactly opposite sides of a central point.
All crystals can be classified into seven crystal systems:
These seven crystal systems are classified in terms of:
- Crystallographic Axes: imaginary lines of reference running through the crystal and intersecting in the center at a fixed point called the origin.
- Axes of Symmetry.
- Planes of Symmetry.
- Center of Symmetry.
|Cubic System:||The most symmetrical of all the crystal systems, it possesses:|
Acicular: Slender needle-like crystals.
Botryoidal: Resembling a bunch of grapes (e.g Malachite).
Crystal Face: One of a number of flat surfaces bounding a crystal.
Form: This is a group of similar crystal faces.
Lamellar: When a mineral is composed of thin layers or plates, like the leaves of a book.
Pinacoid: This is a pair of crystal faces which are parallel to two crystal axes and cut by the third.
Basal Pinacoid: Parallel to the lateral axes.
Prism: A crystal form whose faces are parallel to the principal axis and cut by the lateral axes.
First-order: Each prism is cut by two lateral axes.
Second-order: Each face is cut by only a single lateral axis.
Striations: Growth lines on a crystal face.