Properties of Gemstones

Origin of Gemstones

Most gemstones are mineral crystals and can be formed in various ways. Some crystallize from molten magma and gases of the earth's interior, and some from volcanic lava steams that reach the earth's surface. As it slowly cools, it forms what is known as igneous rocks. Other minerals crystallize naturally from hydrous 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 lower regions of the earth's crust. This is known as metamorphic rocks.

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 pointed object. Mohs set up a comparison scale using ten minerals of different degrees of hardness, which ranges from 1-10(1 being the softest and 10 the hardest), which is still in use today. Gemstones of scratch hardness 1 and 2 are considered soft, those of 3 - 5 medium hard, and those over 5 are hard.

Cleavage and Fracture
The cleavage of a gemstone is the manner in which minerals can separate and split off. Gemstones can "cleave" along certain planes where the atomic bonding is weaker. Cleavage is often used to divide large gem crystals or to remove faulty or uneven pieces. The largest diamond ever found was the Cullinan Diamond, which was 3,106 carats in 1908 and was cleaved into 3 large pieces then cleaved again into numerous smaller pieces.

When dealing in gem trade, there are four main units of measurement 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 is equal to 100 points, so a point would be equal to.01 or one-hundredth of a carat. The gram is a unit of weight measure used for less precious/expensive gemstones and especially for rough stones. The grain is a unit of weight measure for pearls and is equal to.05g or ¼ of a carat. The momme is an old Japanese measure for cultured pearls. One momme is equal to 3.75g or 18.75ct.

Density and Specific Gravity
Another way to measure gemstones is to measure the specific gravity. The specific gravity is the measurement of weight of a specific item, in this case a gemstone, which is of any volume, compared to the weight of an equal volume of water. Specific gravity is usually expressed in density, which is the grams per cubic centimeter(g/cm3) or for larger gemstones kilograms per cubic centimeter (kg/cm3). To determine density, there are two methods that have been proven to be successful. The first method is the buoyancy method which can be done 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 fluid that is displaced by the object. The second method of determining density is the suspension method. The suspension method is states that an object will float in liquid of higher density, sink in a liquid of lower density and remain suspended in 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.

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, some 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 in 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 can have its own inherent color from its chemical makeup. All other colors are created by impurities. The inherent color can be seen by streaking the mineral on a rough porcelain plate called a streak plate.

Refraction of Light
The amount of light that refracts in each gemstone is constant in each specific gemstone. This can be used to identify each specific stone. The amount of light that refracts through a specific stone is called the refractive index(RI). A refractive index(RI) can be properly defined as the measure of the angle at which light traveling through a stone is bent or refracted. The refractive indexes of gemstones are between 1.4 - 3.2. In most gemstones, the exception of opals and glasses, the ray of light is refracted when entering and is divided into 2 separate rays, each of which is travelling into separate directions. This is what is called double refraction. Light refraction can be measured using a refractometer (link to refractometer on website). Unfortunately testing with common refractometer is only possible to measure the value on 1.81 and only stones with flat stones with flat face or facet are suitable. Refractometers can also detect stones with double refraction.

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 cloudiness or haziness of the gemstone. Transparency can be affected in many ways. One way is through the gemstones inclusions. Certain types of inclusions such as solids, liquids and gaseous inclusions can affect the gemstones ability to transmit light. This will be discussed more on the next portion.

Most gemstones are completely "clean", meaning 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 gaseous. Solid inclusions may include some minerals of same or foreign. This may have been caused from older minerals can be surrounded by a host stone. Liquid inclusions are usually in the form of water but may also include saline. Gaseous inclusions can different types of gasses such as oxygen, nitrogen, etc. Some inclusions may include both liquid and gas and is called a two-phase inclusion.

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