Gemstones: Introduction
For the last 10,000 years, mankind has always been fascinated with gemstones. Its colors, properties, myths and legends are diverse and captivating. In ancient times, gemstones were reserved only for the wealthy and served as status symbols. They were also used as amulets and talismans, offering protection against ghosts, repelling evil, and preserving health. Today these stones can be admired in museums; treasure vault, and are even purchased for jewelry. Next, we will discuss more about gemstones, their origins, and properties.

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Origin of gemstones
Most gemstones are mineral crystals and can be formed in a number of ways. Some crystallize from molten magma and gases from within the earth, and some from volcanic lava vapors that reach the earth’s surface. As it slowly cools, it forms what are known as igneous rocks. Other minerals crystallize naturally from hydrated solutions or grow with the help of organisms. This is known as sedimentary rocks. Gemstones can also be formed by recrystallization of existing minerals under great pressure and high temperatures in the lowest regions of the earth’s crust. This is known as metamorphic rocks.

gemstone properties

hardness scale
Different gemstones can be differentiated by their hardness or scratch hardness. The Viennese mineralogist Friedrich Mohs introduced the term scratch hardness, which is the resistance of a mineral when scratched with a sharp object. Mohs established a comparison scale using ten minerals of different degrees of hardness, ranging from 1 to 10 (1 being softest and 10 being hardest), which is still in use today. Gemstones with scratch hardness 1 and 2 are considered soft, those 3 to 5 are medium hard, and those over 5 are hard.

Scratch Hardness     Mineral used Cutting

(Mohs)
1.                                 Talc                 0.03

2.                                 Gypsum             1.25

3.                                 Calcite            4.5

4.                                 Fluorite         5

5.                                 Apatite           6.5

6.                                 Orthoclase     37

7.                                 Quartz             120

8.                                 Topaz               175

9.                                 Corundum         1,000

10.                               Diamond           140,000

Excision and fracture
The cleavage of a gemstone is the way that minerals can be separated and divided. Gemstones can “cleave” along certain planes where the atomic bond is weakest. Cleavage is often used to split large gem crystals or to remove defective or irregular pieces. The largest diamond ever found was the Cullinan diamond, which was 3,106 carats in 1908 and was split into three large pieces and then split again into numerous smaller pieces.

Weight
When it comes to gem trading, there are four main units of measurement that are used as units of weight. There is the, carat, gram, grain and momme. The carat is the basic unit of measurement for gemstones. It is equal to 200 milligrams or .2 grams. The price of a certain gemstone would be indicated by the total number of carats. To measure smaller diamonds, it is often expressed as points. The way to look at points is that 1 carat equals 100 points, so one point would equal 0.01 or one hundredth of a carat. The gram is a unit of weight measurement used for less precious/expensive gemstones and especially rough stones. The grain is a unit of measurement of weight for pearls and is equal to 0.05 g or ¼ carat. The momme is an old Japanese measure for cultured pearls. One momme equals 3.75 g or 18.75 ct.

Density and Specific Gravity

Another way to measure gemstones is to measure the specific gravity. Specific gravity is the measure of the weight of a specific item, in this case a gemstone, having any volume, compared to the weight of an equal volume of water. Specific gravity is usually expressed in density, which is grams per cubic centimeter (g/cm3) or, for larger gemstones, kilograms per cubic centimeter (kg/cm3). To determine the density, there are two methods that have proven to be successful. The first method is the buoyancy method which can be performed with the help of a hydrostatic balance. The hydrostatic balance uses Archimedes’ principle of buoyancy, which states that the buoyant force of an object in water is equal to the weight of the fluid that is displaced by the object. The second method of determining density is the suspension method. The suspension method states that an object will float in a liquid of higher density, sink in a liquid of lower density, and remain suspended in a liquid of the same density. This is an inexpensive method that can be performed using a set of liquids where the amount of density is known.

Color
Of all the properties of gemstones, color is the most important. This is because color is the most obvious and attractive feature of gemstones. The color of the gemstone is determined by a mixture of different colors of light. When light passes through a material, part of the wavelength is absorbed, while the rest passes through. If all the light is absorbed, the stone will appear black. If all wavelengths are absorbed to the same degree, the stone will be white or gray. Most gemstones cannot be distinguished by color alone because many of them have the same color. Gemstones of the same species can vary in different colors. A certain gemstone may have its own inherent color due to its chemical composition. All other colors are created by impurities. The inherent color can be seen by scratching the mineral on a rough porcelain plate called a scratch plate.

light refraction
The amount of light that is refracted by each gemstone is constant for each specific gemstone. This can be used to identify each specific stone. The amount of light that is refracted through a specific stone is called the refractive index (IR). An index of refraction (IR) can be correctly defined as the measure of the angle at which light traveling through a stone is bent or refracted. The refractive indices of gemstones are between 1.4 and 3.2. In most gemstones, with the exception of opals and crystals, the light ray is refracted as it enters and splits into 2 separate rays, each traveling in separate directions. This is what is called double refraction. Light refraction can be measured with a refractometer (link to refractometer on website). Unfortunately, testing with a common refractometer is only possible to measure the value at 1.81 and only stones with flat or faceted faceted stones are suitable. Refractometers can also detect double refractive stones.

Transparency
Another way to evaluate gemstones is through their transparency. The transparency of a gemstone is the property of transmitting light through a gemstone without any haze or cloudiness of the gemstone. Transparency can be affected in many ways. One way is through gemstone inclusions. Certain types of inclusions, such as solid, liquid, and gas inclusions, can affect a gemstone’s ability to transmit light. This will be discussed further in the next part.

inclusions
Most gemstones are completely “clean”, which means that they may contain some type of foreign material or some irregularity in the gemstone. This is what is called an inclusion and is one of the ways that makes a gemstone unique. Inclusions can come in 3 different types: solid, liquid, and gas. Solid inclusions may include some of the same or foreign minerals. This may be because older minerals may be surrounded by a host stone. Liquid inclusions are usually in the form of water, but can also include saline. Gaseous inclusions can contain different types of gases such as oxygen, nitrogen, etc. Some inclusions can include both liquid and gas and is called a two-phase inclusion.