There are numerous gemstones so there is a big challenge in identifying the gemstones. Important information that will help everybody is in properly understanding the physical and optical properties of these gems. To start with, we have to understand that gems can be classified into two categories.
The Categories of Gems
1. Organic gemstones can be identified as organic remains of a past life. This includes gemstones such as pearl, amber, and so on. They are amorphous and do not have a crystalline form.
2. Inorganic gemstones on the other hand are mineral gemstones that have definite chemical compositions, a characteristic atomic structure, and a crystalline form.
As the title suggests, this article will focus on the inorganic gems and their characteristic structure.
We already know that these inorganic minerals have a unique chemical composition. These inorganic substances mostly grow in a symmetrical form. Usually they have a characteristic external shape that is a result of the orderly internal arrangement of atoms.
Now these inorganic minerals can be further classified into two sections
1. Euhedral: Most of these minerals have developed over the years inside the Earth, and so, some of them have developed and displayed proper crystal symmetry.
2. Anhedral: Alternately, there are some minerals that are not so well developed but are still crystalline. They are classified as Anhedral crystals.
You have to always keep in mind that minerals have a characteristic crystalline structure. In other words, they are built with a basic structure that keeps on repeating itself in a very regular and symmetrical way. This crystalline in turn has an impact on certain physical properties of these gems like the hardness, cleavage, fracture, specific gravity, and also the external shape.
But before we move on, one more thing that will help you always in understanding a gemstone jewelry is that properties of these stones are influenced both by their crystal structure as well as their chemical composition.Are you finding this confusing? Let us understand this in more detail – this should clear your doubts. As we have already mentioned, these minerals have a specific chemical composition. These elements and ions present inside the mineral determine how they will be arranged. This in turn will affect the shape of this mineral. However, the arrangements are also influenced by what we know as symmetrical restrictions. So, these are the two elements that decide the final shape of the gem. You might be wondering why two sets of minerals for example graphite and diamond have the same composition, but still differ in appearance. This happens because of differing temperatures and pressures. This creates different arrangements of elements that in turn have very contrasting properties.
Depending on the crystalline structure, minerals can be classified into six categories.
These categories are based on imaginary crystallographic axes and the angles between the axes.
1.Isometric system: Here there are three equal crystallographic axes. The angle between the axes is 90 degrees. The typical crystal shapes that fall under this category are cubes, octahedrons, and dodecahedrons.
2.Tetragonal system: This system again has three axes at right angles, but the difference from the previous system is that, instead of all the axes of the same length, here the third axes can be longer or shorter than the other two. Examples of this system are four-sided prisms and pyramids.
3.Orthorhombic system: In this case, the angle between the axes is still 90 degrees, but the lengths are not the same. The characteristic examples include rhombic prisms.
4.Hexagonal system: Here we have four crystallographic axes. Three of them are in one plane and same length. The angle between them is 120 degrees. The fourth axis on the other hand is perpendicular to the plane where we have the other three axes. Also it has a different length than the other three. Example is hexagonal prisms.
5.Monoclinic system: Here again, we have three axes but they are all of different lengths. The angle between the two axes is 90 degrees. The third one can have a varying angle. Typical forms are pinacoidal (paired faces).
6.Triclinic system: Again we have three axes, but there is differing length. The inclination from each other can also vary. This system is the least symmetrical. The shapes are usually pinacoidal.
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