On Earth, there are about 3,500 different minerals out of which 120 are defined as "gemstones" and only about 50 of them are suitable for mounting in jewelry. All gemstones have one collective name "crystals" – minerals with a uniform crystalline structure – the same geometrical crystal lattice and specific well defined chemical composition.
A. Formation of Gemstones in Nature
a. Volcanic activity such as the eruption of a volcano.
b. Changes in the Earth, such as an earthquake
c. Precipitations which seep into the depths of the earth and create geological changes
Evaluation of Gemstones
The four main criteria by which gemstones are evaluated are known as the "4 C's":
Color, Clarity, Carat, Cut
Color – The color of the stone is one of the most important criteria. There are essential differences between the colors around the outside of the crystal and the colors in its center. The more one penetrates into the center of the crystal, the color becomes richer, clearer and much more impressive.
Clarity – Measured by a 4-Grade Clarity Scale
Grade 1 is given to a transparent stone, the highest degree of clarity, while Grade 4 means a completely opaque stone. Transparency is measured in relation to the type of stone.
Inclusions – The center of the stone is also examined as part of the clarity evaluation – dirt, rhutiles, different minerals which have crystallized together with it. With gemstones, the aim is to find stones with greatest clarity and without inclusions. In the picture are two amethysts, one clear and one rich with inclusions.
There are a number of minerals in which the inclusions increase their value. For example, amber (the more inclusions found in this stone, the greater its value) and Roman glass (the more colorful its patina, the greater the value of the glass).
Carat – Weight: While with gold, the carat is a measure of its purity, in gemstones it is a measure of weight. "Carat" comes from the Greek word "keration", meaning "fruit of the Carob". In the past, merchants from the east used carob seeds as counter weights for weighing gemstones that were brought usually from the east to the west. Carob seeds were unique in that they are all of equal size and weight – 0.2 gram. Hence the definition of the weight measure:
1 carat = 0.2 gram
Cut – Cutting / Polishing: The type of cutting and quality affects the value of a gemstone.
1. Cabochon Cut – smoothed, rounded surface
2. Rose Cut – flat upper surface, sides cut at different angles
3. Faceting Cut – all the surface is cut at different angles to form a dome. This is the most prestigious cut - the more facets there are, the higher the value of the stone
4. Marquis Cut – eye-shaped
5. Octagon Cut – rectangular-shaped with eight angles around the sides, also called Emerald Cut, the most common gem cut