The properties of a gemstone are basically classified under two divisions: physical and optical. While physical properties are concerned with what a gem is made of and how it is made, the optical classification deals with how a gem interacts with light. In this article, we’ll focus on the physical properties.
The physical properties of a gemstone are not affected by how it reacts to light falling on it. On the contrary, every property of a gemstone, optical or otherwise, depends on the material it is made of and the arrangement of atoms in its structure. That is why, an amber bead necklace looks very different from a shiny obsidian pendant, although both of them are gemstones.
Amber and obsidian, or any two kinds of gemstones for that matter, are different because they have different chemical compositions and three dimensional structures. On the basis of structure, gemstones can be divided into two groups – crystalline and amorphous.
Crystalline: A crystalline gemstone has a highly organized and predictable crystal lattice structure, and a specific chemical formula. As a result, they have more brilliance when chiseled in a geometrical shape. Diamond, rubies, emeralds, amethyst have this kind of internal structure.
Amorphous: The word is derived from the Greek term “amorphos,” meaning shapeless. Though an amorphous gemstone is not devoid of structure entirely, it is relatively less defined and somewhat haphazard. Amber, jet, opal, natural glasses, and metamict minerals fall in this category.
X-ray diffraction is the method used to study the internal crystal formation details. It is similar to the X-ray procedure commonly seen in a medical set-up. While studying gems, X-rays are passed through them to fall on a sensitive film. Like more X-rays are absorbed by dense bones, dense clusters of atoms absorb more X-rays while scant areas allow them to pass through and impact the film. The high contrast picture that is produced can be read and interpreted to reveal information about structure patterns. With research using this method, it has been found out that crystalline gemstones can be further separated into single crystals and aggregate gems.
Single crystals: Stones like pyrites and amethysts are made of single crystal units of one kind visible through a microscope.
Aggregates: If the single crystal can be imagined to shrink multiple times its size and then thrown together in random orientations, it will give rise to aggregates.
Aggregates can have starkly different physical features and behavior than single crystals. For instance, amethyst and chalcedony are the species of quartz – each having crystal of the trigonal system (whatever be their size) and the same chemical formula, SiO2. While amethyst is transparent and of a single color, chalcedonies can be translucent to opaque and sometimes of various different colors. Furthermore, it has been noticed that aggregates are considerably tougher than single crystal stones. Single crystals can be humongous – as big as a truck. For instance, the Crystal Cave in Put-in-Bay, Ohio, which is actually a huge underground geode (a hollow cavity of rocks usually lined with crystals) has walls and ceilings composed of celestite crystals.
Aggregate gems can be further divided into two groups: microcrystallines and cryptocrystallines.
Microcrystallines: They are so called because crystals falling under this group can be resolved under a light microscope. A thin sliver of the crystal is obtained and examined under about 100 to 200 times magnification. Jade is an example of such crystals.
Cryptocrystallines: “Crypto” means one that is not easily revealed. And so, one cannot see the minute crystals of cryptocrystallines under an ordinary light microscope, one would need an electron microscope or specialized polarized lighting to be able to view the minutest crystals. Agate, jasper, chalcedony come under this group.
Another parameter of classification is the composition. Although most crystalline and amorphous gemstones have one mineral in the composition with a little inclusion here and there, there are some gemstones that are have two or more minerals as substantial ingredients of the composition. Such gems are called gem rocks. The most familiar example would be lapis lazuli, a fascinating rock made of lazurite, Hauyne, sodalite, calcite, and pyrite. Other examples would be unakite and Chinese writing stone.
These were the basic parameters of classification of gemstones on physical parameters. In the world of gemology, there are many wonderful gems, and equally wonderful methods to study them. If you liked reading this, you would certainly like to explore more about them.