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).**
[Not classed as organic gems: petrified dinosaur bone agate, cabochon cut from a fossilized coral colony]
Although none of the molecules from the living organisms remain in certain types of organic gems, such as the calcareous corals, the minerals they are composed of were secreted, originally, by the living things as they grew, not replaced later by petrifaction. Likewise, although substantial geologic changes have altered the properties of jet and amber, the materials still consist primarily of the original organic molecules.
[Classed as organic: calcareous "angel skin" coral carved beads, carved jet earrings, circa 1925 amber and jet cigarette holder: Image courtesy of www.fraleigh.ca]
A gem is beautiful. Beauty, of course, is a subjective concept that has many aspects, and differs from viewer to viewer, but in general, the attributes of gems which excite our sense of beauty include, color, transparency, luster, brilliance, pattern, optical phenomena and, in some cases, distinctive inclusions.
[Kunzite : color, transparency, brilliance, jasper: color, pattern, luster]
[Ammolite: color, luster, iridescence (an optical phenomenon), rutilatedquartz: transparency, distinctive inclusions]
A gem is rare. There are two types of rarity involved: relative and inherent.
Relative: Many gem minerals occur in various locales and, often, in large deposits, but the vast majority of the material does not approach "gem quality".
Inherent: Other minerals occur in only a few locations or in very small deposits. Inherently rare gems are doubly rare as the fraction of an already small amount of ore which is gem quality is very, very, small indeed.
[Ruby: a gem with relative rarity, Benitoite蓝锥矿: a gem with inherent rarity]
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The mineral corundum (of which ruby is a gem example) is widespread and abundant. So much so, that an enormous amount of low grade corundum is used in industry for abrasives, due to its hardness (9 on a scale of 10). [Interestingly, very tiny, non-gem grade, corundum crystals have found use in today's beauty industry-->as the active ingredient in both medical "dermabrasion"磨去皮肤疤痕agents, and over the counter "exfoliating片状剥落" products.]
Benitoite, on the other hand, is found in gem quality in only one location on Earth: the San Benito River Valley in California. Only a few ounces of cut gems result from each year's mining efforts, almost all of which are quite small in size. Ironically, this ultra-rare, nearly unobtainable stone has been officially designated as the State Gemstone of California.
[Pyramid of gem rarity]
Usually in a deposit of gem mineral bearing ore, the majority is not the mineral being sought. From the small portion of the ore which bears the gem mineral, the majority is too low grade to have any gem uses. For example, 80% of the diamond recovered from diamond bearing ore, is industrial grade.
Within the small amount of gem grade material, the bulk of it is of lowest quality and useable only for inexpensive beads or trinkets小装饰品. The even smaller amount of better material which can be extracted, is mostly middle grade, or that which is used for cabochons and better beads and carvings. A tiny fraction is high grade and can be used for faceting. Most of the facet grade material has some defects in color or clarity that limit it to "commercial" quality gems. Only the most miniscule part of the original deposit is top grade: AAA color and flawless clarity.
Because the starting amount at the base of the pyramid for a gem like ruby is much larger than the starting amount for a gem like Benitoite, the amount at the top is correspondingly larger.
Finally, picture taking that top "highest grade" part of the pyramid and dividing it, again, into layers based on size: from small at the base, to large at the tip. Is it any wonder that the largest, finest gems bring astronomical prices?
Speaking of prices: How valuable are gemstones? If you ask people at random to name a valuable commodity, many might say gold. And true, we do think of gold as valuable. Consider this:
· Good quality amethyst gems sell for about $40/ct
· Fine quality aquamarine海蓝宝石 sells for around $200/ct
· Highest gem quality blue sapphire sells for as much as $2500/ct.
Pure gold, however, is worth well under $10 per carat! Down through the centuries, gemstones have respresented the ultimate in portable wealth. (In the next lesson, we'll go through the calculation that produced the cost of gold figure).
A gem is durable. It must be strong enough to withstand the stresses and forces involved in fashioning it, and its subsequent use as an ornamental object, or in jewelry. Most everyone has heard of "hardness" and knows that harder is better, in terms of using gems for jewelry--> but in reality, hardness is only the beginning of the story. There are two other aspects of gem durability that are at least as important as hardness.
Three Aspects of Durability
1) Hardness is the ability to resist scratching. Commonly measured on the "Mohs" Scale of 1 - 10. Talc lowest (1), diamond highest (10). Soft gems, especially those below 7 will tend to become dull through abrasion with harder materials in the environment, and lose their surface polish and their crisp edges over time.
2) Toughness is the ability to resist breaking or chipping. This property is measured in relative terms rather than on a numeric scale: sphalerite is fragile, diamond is moderately tough and jade is exceptionally tough. The lower the toughness of a gem the more susceptible易受影响的 it is to damage by the kinds of blows and knocks that are inevitable with frequent wear and use.
3) Stabilty is resistance to changes caused by environmental factors such as temperature, chemicals and light. Apatite is temperature sensitive, pearls are chemically sensitive, and Kunzite's color is unstable in strong light. Unstable gems exposed to common factors of the natural or man-made environment are likely to break, change color, or lose their luster.
Food for thought: Taking into account each of the aspects of the definition of a gem, explain why is each of these not a gem. (Answers to the question are found at the end of the lesson.)
Question One: An exotic butterfly wing, an industrial grade natural diamond, quartz beach sand, a laboratory grown ruby.