Alpaca Silver Jewelry

Explanations of Gem Properties – Pleochroism

I thought you might be interested in explanations for the properties of gemstones of which I have been writing about.  Just seeing the Crystal structure names or specific gravity numbers of a gemstone probably, if you’re like me, doesn’t mean a lot to you.  But an explanation of what Crystal structure or specific gravity is and why it is important to the each gemstone might make it more interesting.  I hope so, because I am going to provide explanations for these properties in the next few postings so, I believe you can get a better understanding of gemstones in the future.

After an introduction I will explain as fully as I can only one specific property per posting.  As you know there are many property characteristics to describe each gemstone and the post would become very large and burdensome if I were to try and put them in one posting.  The first posting will an introduction and the explanation of the property – Pleochroism.

Introduction

If you are like most Americans, at some time, you have played the guessing game “Animal, Vegetable, or Mineral?” The premise behind the game is that one individual receives a certain number of questions in which to guess what material object another person has chosen. Of course, the first question is that every object that we can think of is supposed to fall within one of these three broad classification schemes. However, this is not always true. One problem is that some objects fall into 2 or more of these categories (some viruses have a Crystalline stage; some animals undergo photosynthesis, etc.). However, the idea behind the game is that everything made of matter will fall into a classification of either living or non-living, with minerals being the catch all for non-living.

The other problem with the game is that a mineral is much more than just a non-living object. What it is exactly, though, will raise a debate amongst geologist. A check of different textbooks will find many different definitions for mineral. For the purposes of this blog, we are going to define a mineral as a substance that is naturally occurring, inorganic, Crystalline in nature, and have a definite chemical make-up. The first of these criteria means that anything man-made is not considered a mineral. This is somewhat problematic; as mankind has developed ways of creating certain gemstones in the lab that are almost indistinguishable from their natural counterparts. For instance, industrially created Diamonds are used for many different tools, such as Diamond-tipped saw blades. The second criterion also has its problems. Certain minerals such as Graphite, Diamonds, and Calcium Carbonate can and do have biological origins. Graphite and Diamonds can come from plant matter. Calcium Carbonate is the chemical that makes up seashells. By convention, they are usually included amongst minerals.

The third and fourth criteria are less problematic. The fact that a mineral must have a Crystalline structure eliminates all liquids. It also eliminates all glasses, as these are amorphous solids with no definite atomic arrangement. The chemical make-up does come with one caveat: some minerals are allowed to have substitutions of certain chemicals in their molecular structure. As an example, Hornblende is a complex mixture of Hydrous Ferromagnesium Silicate that contains various proportions of calcium, aluminum, and sodium within it. These substitutions usually just change the color of the mineral and do not radically alter the other properties of the mineral.

Gemology is the science, art and profession of identifying and evaluating gemstones. It may be considered a branch of mineralogy. Some jewelers are gemologists and are qualified to identify gems.

With further training they can become appraisers (evaluators). There are professional schools and associations of gemologists and certification programs.

Some gemologists specialize in various gemstones, such as Diamonds or Emeralds. Such specialists are found in fields in which a great deal of knowledge is required, especially in the determination of exceptionally “pure” gems.

Recently, the demand for gemological services has grown, as increasing quantities of synthetic gems such as Cubic Zirconia and synthetic Moissanite are manufactured. Gemologists perform such work as the identification of synthetic and natural gemstones, fracture-filled gemstones, and color-enhanced or treated natural gemstones. Gemology in the 21st century has become a rigorous science where constant study is necessary for correct identifications.

Gemstones are basically categorized based on its Crystal structure, specific gravity, refractive index and other optical properties.

Gemologists study these factors while valuing or appraising cut and polished gemstones. Gemological microscopic study of the internal structure is used to determine whether a gem is synthetic or natural by revealing natural fluid inclusions and evidence of treatments to enhance color.

The spectrum analysis of cut gemstones also allows a gemologist to understand the chemical structure and identify its origin as it is a major factor in valuing a gemstone.

For example a ruby from Burma will have definite internal and optical activity variance as compared to a Thai Ruby.

When the gemstones are in a rough state, the gemologist studies the external structure; the host rock and mineral association; and natural and polished color. Initially the stone is identified by its color, specific gravity and its place of origin.

Property Pleochroism

Pleochroic minerals are minerals that show different colors depending on what direction the viewer is observing the crystal. The effect is sometimes quite dramatic. Many minerals are technically pleochroic, but more often than not the color change is so small that it requires optical instruments to detect it. However there are some minerals that show an incredible color change. The greatest change is limited to three colors and is called trichroic. A two color change is called dichroic. Pleochroic, which means “many colors”, is often the term used to cover both. Many times the color change is limited to shade changes such as from pale pink to dark pink. In order to view pleochroism you need an individual transparent crystal.

Pleochroism is caused by the absorption of different wavelengths of light travelling through different directions in the crystal. If in one direction, all wavelengths but yellow and blue are absorbed then the crystal will be green (yellow and blue make green). If in another direction all wavelengths are absorbed but yellow, then the crystal will appear yellow. If the crystal is turned from the first direction to the other, then it will change its color from green to yellow. Isometric minerals cannot be pleochroic since they have the same structure and thus the same light absorbing capabilities in all directions. Tetragonal, trigonal and hexagonal minerals can only be dichroic since they have one unique structural direction along the major symmetry axis and one direction in every other direction. Only orthorhombic, monoclinic and triclinic minerals can be trichroic since they have three unique axes of symmetry and therefore three unique directions that can absorb light in three different ways. Although pleochroism is a rare phenomenon to easily observe, it can be diagnostic.

Some of the best examples of pleochroism are found in specimens of:

Andalusite – Yellow-green to red brown

Carletonite – Colorless to blue

Elbaite – Pale color to a darker color

Iolite – Colorless to blue to purple

Kunzite – Pale pink to dark pink

Tanzanite – Colorless to blue to purple

Zoisite – Colorless to pink to yellow

Pleochroism is defined as:

None (no variation in color) – Isotropic minerals are always dark under crossed polarizers. Anisotropic minerals are not. If no color variation is observed on rotation under plane-polarized light then the mineral is non-pleochroic.

Dichroic (two colors observed) – Dichroic minerals are generally always hexagonal, trigonal or tetragonal.

Pleochroic (three colors observed)—Pleochroic minerals are generally always orthorhombic, monoclinic or triclinic.

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