PHYSICAL AND  OPTICAL  PROPERITES OF GEMS

Properties of Gemstones: There are two sets of characteristics possessed by every gemstone, and by which they are studied, identified and evaluated:

1) physical properties, and 2) optical properties.

In this lesson we'll be concerned with physical properties: those which do not depend on the gem's interaction with light to be expressed or measured. (In the next lesson we'll look at the optical properties).

All properties of gems (whether physical or optical) derive from the underlying three dimensional structure and chemical composition of the gem. Or to put it another way, the chemical elements that make up the gem, and how the atoms of those elements are put together to create its inner structure, determine all those properties that we can see, feel, and measure.

Amorphous vs Crystalline: The most basic discrimination that can be made, based on internal structure, is that between gems which are amorphous, and those which are crystalline.

Crystalline gems have a specific chemical formula, and a well defined, highly predictable internal structure, known as a crystal lattice. Amorphous gem species also have a specific chemical formula, but their constituent atoms are not arranged in such regular and predictable patterns as those of crystalline materials. 

Amorphous Gems

"Amorphous" literally means "without form", but, of course, these materials have a form--> it's just not highly regular and predictable, nor is it expressed outwardly by the formation of crystals. Some examples of amorphous gems are: the natural glasses, amber, jet, opal, and "metamict蜕晶质 " minerals.

Natural Glasses: The atoms making up a glass (either natural or man-made) have been cooled from the molten state so quickly that they fail to assume a regular crystalline pattern. A volcanic glass, like obsidian, then, might be formed if a volcano released lava into the air or water such that it was very rapidly cooled--> this very same lava could, upon slower cooling, form a crystalline material (like basalt, for example).

Obsidian ranges in color from light yellow through brown to black and can be transparent, translucent, or opaque. Those of our ancestors, who lived in areas of volcanic activity, made ready use of these natural glasses.

[Obsidian artifacts]

In some cases, due to the presence of other minerals with different crystallization temperatures, when the molten material cools, crystal inclusions may be formed. These can give the obsidian an interesting pattern, or affect the structure in such as way as to cause an optical phenomenon, like iridescence 彩虹色 . Although most obsidian is drab 单调的;土褐色的 , single-color translucent material, two interesting and more showy forms of this volcanic glass can be seen below:

["Snowflake" obsidian, "velvet" obsidian]

Tektites熔融石,玻陨石 : Another group of natural glasses, known collectively as "tektites", are not found associated with volcanic eruptions, but rather in places which are believed to have been sites of meteoric impact. The heat and compression of the impacts are thought to have melted silica sand, and the molten bits which were flung into the air rapidly cooled into their glassy state.

[Tektite from China]

Although, like most obsidian, the various types of tektites are dull colors of green and brown, they are still much sought after by gem and mineral collectors. They have a following as well among those who ascribe mystical properties to these gems, perhaps because of their association with celestial events.

The most commonly seen tektite is a green, near transparent type found in the Moldau River Valley in Eastern Europe, known as Moldavite绿玻陨石 . An intriguing light yellow form of natural glass has been found in several areas within the Libyan Desert, and, to date, has not been associated with a meteor impact, so its origin remains uncertain.