Characteristics of Gemstones

Optical Phenomena

Some stones have characteristics of optical phenomena which is caused by how light reflects off of the stone's surface or inclusions within the stone depending on the chemical composition, or the arrangements of inclusions within the stone.

Inclusions

click for large image		Inclusions are minerals, insects, tiny bubbles, or other things which are inside the stone when it was formed in the Earth. Sometimes inclusions are very tiny, needle-like minerals in a contrasting color inside a transparent stone. The green Prehnite has some black tourmaline needle inclusions.
Druzy

click for large image		A Druzy is a stone with tiny crystals formed on it as a result of seepage of silica water during its formation. One side is the rock upon which the crystals sit, and the other side is usually an open cavity inside a geode.
Adularescence

click for large image		It is a bluish-white optical effect which appears to come from inside the stone. This is caused by the inner structure of the stone which causes light to diffract. It appears as a floating sheen billowing across the stone on the inside. Moonstone is an example.
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Aventuresence

click for large image		This is a colorful glittering effect in a stone caused by tiny amounts of mineral inclusions such as hematite or fuchsite. It comes from a reflection of light by mineral inclusions, or glitter. Examples are Sunstone (left) or Celestine (right).
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Iridescence
Rainbow-like colors produced when light reflects off the surface or internal structures of a stone.
Labradorescence

click for large image		Similar to Adularescence, except multiple colors are displayed. The stone Labradorite shows blue, green, yellow, and orange when light is refracted.
Orient

click for large image		This is the reflection of light off of the surface such as the nacre of a pearl. An example is Mother of Pearl.
Chatoyancy
Chatoyancy is the reflection of light off of parallel thread inclusions.
Simple

click for large image		Examples are Seraphinite or Kyanite.
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Cat's Eye
The threads are aligned in one axis, one place, preferably in the middle of a cut specimen. This shows up as a pale streak. An example is Cat's Eye Chrysoberyl.
Asterism
The threads are aligned on two or more axis, crossing each other. An example is a stone with the name "star" such as Star Sapphire.

Transparency or Clarity

Transparency is how much light passes through the stone. A stone with excellent clarity has no visible features inside of it or inclusions, such as an insect that was trapped inside when amber was formed, or crystals different from the larger stone trapped inside when the stone was formed in the Earth.

Transparent

click for large image		A high portion of light travels all the way through a transparent stone. An example is Rock or Crystal Quartz and Fluorite.
Translucent

click for large image		Some but not all light passes through the stone. This is different from transparent in which light can pass all of the way through a stone. Examples of translucent stones are Prehnite (left) and Fluorite (right) (low translucency).
Opaque

click for large image		No light passes through the stone. Examples are Coral (left) or Snow Quartz (right).
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Pleochroism

click for large image		This is the ability of some crystals in a gemstone to handle light differently in different orientations. This results in the gemstone displaying different colors when the stone is turned in different directions. An example is tanzanite. Note the arrows, each showing different colors depending on where the light hits the stone; if all of the stones are oriented the same way, there would be no discernable difference in the color.1-a more intense color, 2-typical color, 3-a grayish color.

Luster

Luster is the quantity and quality of light reflected from the surface. It depends on the condition of the surface (smooth, pitted, fractured), the degree of polish, and how the light is 'bent' as it strikes the surface (refractive index). Some lusters are described as:

Adamantine
Diamond
Dull

click for large image		Rock
Glassy (or Vitreous)

click for large image		Crystal Quartz
Greasy

click for large image		Peridot (Kind of hard to see the greasiness in the photo.)
Pearly

click for large image		Cultured Pearls
Resinous

click for large image		Amber
Silky

click for large image		Larimar
Waxy

click for large image		Turquoise (It's kind of hard to see the 'waxiness' in the photo.)

Color


click for large image		Color of a gemstone is an obvious characteristic. Some stones come in only one color; others come in several different colors. Color may be solid or patterned.
Common patterns are Banded with more or less linear parallel lines of color such as Agate, or Orbicular with color in small circles such as Ocean Jasper.
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