What is a gem?

Although you might not be able to give a precise definition of a gem at this time, few of you would have any trouble in recognizing that the images below are of gems. So then, what characteristics do they exhibit that allow you to intuitively recognize them, and cause gemologists or geologists to officially label them as such?

 

[Cabochon and carved gems, faceted gems]

A gem is a natural, mineral or organic substance, that has substantial beauty, rarity, and durability. Let's take each underlined part of that definition and examine it:

Natural means that the material was not made, or assisted in its making, by human effort. When such is the case, modifiers such as "laboratory grown", "synthetic", "cultured", or "man-made", must, by Federal Trade Commission (FTC) regulations, be used in the descriptions of any such pieces being advertised or marketed. Man-made "gems" have all the chemical, optical and physical characteristics of the natural materials they imitate, but they do not have their rarity or value. You can be certain whenever you see any of the above modifiers that the material in question is not of natural origin.

A mineral can be defined as a crystalline solid with a specific chemical formula, and a regular three dimensional arrangement of atoms. (In a later web lecture, this definition will be broadened to include "amorphous" materials which have a specific chemical formula but do not have a specific crystalline structure, for example, opal and natural types of glass).

Mineral Gems

Iolite, which has a specific chemical formula of: Mg2Al4Si5O18 and a regular arrangement of atoms which places it into a crystal system, with other minerals of similar structure, known as the orthorhombic crystal system is a mineral gem. Another example is emerald, Be3Al2(SiO3)6, a member of the hexagonal crystal system. (The attributes of the various crystal systems will be presented in an upcoming lesson.)

 

[Faceted iolite, uncut emerald crystal]

Organic Gems

An organic gem is one that was made by living things, present or past. Examples include pearls, coral, jet, ivory, shell and amber. Such gems consist of the molecules formed by the organism, although these molecules may have been altered somewhat due to compression or other geological or chemical forces.

 

[Organic gems: coral and freshwater cultured pearl earrings, faceted amber (enlargement showing fossilized insect within the gem]

**Gems such as "petrified dinosaur bone" and many other "stony" fossil gems, are classified as mineral, rather than organic. Although its true that bone is an organic material: the reasoning involved is that the original organic molecules and structures of long ago have been totally replaced with mineral solutions such as silica. (This common geological process is called petrifaction).**