Basic Crystallography

All gemstones can be classified as either:

Amorphous

1. No orderly internal atomic structure.
2. No naturally-occurring characteristic shape.
3. Products of rapid cooling.
4. Physical properties are constant in all directions.

Crystalline

1. Definite & regular internal atomic structure.
2. Geometrical external forms.
3. Directional properties.
4. Products of slow cooling.
5. Identical in all crystals of a given species.

Gemstones can also be:

Massive

A term used to describe a crystalline substance that does not have a naturally-occurring characteristic shape. (i.e. Rose Quartz)

Crypto-crystalline

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)

Habit

The characteristic crystal form displayed by a mineral.

Crystallography

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:

1. Cubic
2. Tetragonal
3. Hexagonal
4. Trigonal
5. Orthorhombic
6. Monoclinic
7. Triclinic

These seven crystal systems are classified in terms of:

1. Crystallographic Axes: imaginary lines of reference running through the crystal and intersecting in the center at a fixed point called the origin.
2. Axes of Symmetry.
3. Planes of Symmetry.
4. Center of Symmetry.

BASIC TERMINOLOGY

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.