Gemmology

Physical, Chemical and Optical Properties

All gemstones can be classified as either:

1. Amorphous
2. Crystalline

Amorphous gemstones are characterized by:

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

Examples:

• Glass
• Amber
• Jet
• Opal

Crystalline

Crystalline gemstones are characterized by:

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.

All crystalline gemstones can be classified into the following crystal systems:

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

The identification of gemstones relies on their unique physical, chemical and optical properties. These include:

Specific Gravity

The weight of a gemstone in air compared to the weight of an equal volume of pure water at 4 degrees Celsius.

Example: If a gemstone weighs 8 carats in air and 6 carats in water the "Specific Gravity" is said to be 4.00 or 4 times its volume.

1 cc at 4 degrees Celsius = 1 gram.

Cleavage

The tendency of a crystalline substance to split parallel to certain definite directions (when force is applied) producing more or less smooth surfaces. It is strictly a directional property and can only occur in crystalline substances. It is due to weaknesses in the orderly placement of the atoms within a crystal. Similar to a grain of wood.

Examples:

• Diamond
• Fluorspar
• Kunzite
• Topaz
• Calcite

Hardness

The ability to resist scratching or abrasion when a pointed fragment from another mineral is drawn across its surface with insufficient force to cause cleavage. The Moh's Hardness scale is commonly used to measure the hardness of gemstones using ten minerals with a pre-determined hardness. Gemstones with higher numbers have the ability to scratch gemstones with a lower number.

Moh's Hardness Scale

1. Talc
2. Gypsum
3. Calcite
4. Fluorspar
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond (140 to 1,000 times harder than corundum)

Toughness

Resistance to crushing or breakage (Jadeite)

Fracture

Describes a break or chip, other than cleavage, on the surface. Occurs in all materials in any directions.

1. Conchoidal - shell-like, concentric rings (Glass)
2. Splintery - long splintery fibres
3. Uneven - broken or uneven surfaces
4. Even - producing flat surfaces that are not noticeably irregular

Refraction

The bending of light when it passes from a rarer medium (Air) into a denser medium (Gemstone)

Single Refraction (Isotropic)

Light passing through a substance is bent from its original path but emerges as a single ray. Only occurs in gem minerals belonging to the "Cubic" crystal system and "Amorphous" material.

Double Refraction (Anisotropic or Birefringent)

Light passing through a substance is split into two rays, which travel at different velocities causing differing amounts of refraction. Occurs in gem minerals belonging to all other crystal systems.

Example: The doubling of the back facets as seen in Zircon.

Transparency

The freedom in which light is transmitted through a substance.

1. Transparent: object appears clear and distinct.
2. Semi-transparent: blurred.
3. Translucent: light transmitted but no object seen.
4. Semi-translucent: light only transmitted through the edges.
5. Opaque: no light allowed to pass.

Lustre

The brilliancy of a stone in reflected light, determined by the amount of incident light reflected from its surface (surface reflection)

1. Metallic - Pyrite.
2. Adamantine - Diamond.
3. Vitreous - Quartz, Ruby, Sapphire.
4. Resinous - Amber, Some Garnets.
5. Waxy - Turquoise.
6. Pearly - Moonstone.
7. Silky - Tigers-Eye Quartz.

Sheen

A shimmering effect due to light reflected from a position inside the stone.

Examples: