Gemstones are the gift from nature, the formation of gemstones in the Earth’s crust takes place in a variety of rock formation.
Rocks
Rocks are naturally occurring aggregates of one or more minerals. There are three different types of rocks, igneous, sedimentary and metamorphic rocks.
Igneous Rocks
Igneous rocks are formed by the solidification of molten magma. Molten magma is usually less dense than surrounding solid rock, as it moves upward to cooler regions of the Earth. As magma loses heat, it cools and crystallized into an igneous rock. These are mainly granite, which is composed a mixture of feldspar, quartz and mica. Many of gem materials are form in below the Earth’s surface is called intrusive or plutonic rocks. Gem materials that can be found in this type of rock include feldspars, quartz, tourmaline, beryl, topaz and zircon.
Sedimentary Rocks
Molten magma can cool on the Earth’s surface, where it has erupted from a volcano is called extrusive or vocalic rocks. Any rocks that are exposed at the Earth’s surface can become sedimentary rocks. These types of rocks are formed when sediments are compressed. The weather breaks down or dissolves rocks into smaller particles. The forces of wind, rain, snow and ice are the form of transportation. These particles, such as sand, eventually come to rest and become hard rock. This type of deposit is called secondary or alluvial deposits. Examples of sedimentary rocks are sandstone, anthracite and limestone.
Metamorphic Rocks
Metamorphic rocks are the result of change in form from existing rocks. Any rocks can become metamorphic rocks. These rocks are created by either heat and pressure or both, for instance, if a rock is buried deep in the Earth’s surface and is subjected to high temperatures and pressures, a new mineral may be formed without magma. Examples of metamorphic rocks include marble, garnet, emerald, ruby, sapphire, andalusite, nephrite and jadeite.
Glass
Most rocks contain cracks, fissures and cavities which allows dissolved chemical compound to be filled. When these high concentrations of chemical compound fills these cracks and cavities, as the result of chemical reaction present in the rock or in solution, ions are join to form orderly patterns, given time and favorable conditions, these ions will slowly build up into chemically uniform solids, known as crystal, and depositing secondary minerals, these type of deposits is known as pegmatite deposits. Some of the gem material pegmatites are tourmaline, beryl and topaz. However, if unfavorable and rapid cooling takes place, ions may not have time to form into patterns, and solidified into a disordered mass or mineral, known as glass, such as obsidian.
Minerals
Mineralogy is the scientific study of minerals, including their composition and how they are form. A mineral may be defined as follow:
(a) is a naturally occurring inorganic solid
(b) is composed of the same substance throughout (homogeneous)
(c) has a definite chemical composition
(d) has an orderly internal atomic arrangement
(e) has physical properties which are constant within narrow limits
(f) its structure is usually crystalline.
Gemstones
Gemstones are minerals with special qualities, which are suitable to render for adornment or decoration. These special qualities are beauty, rarity, durability, and portability.
Beauty cannot easily be quantified as it depends on subjective factors to do with appearance. To display the beauty of gemstones, lapidary has to cut and polished the stone to reveal their color and optical effects to the greatest possible degree. For transparent gemstone, the depth of color and degree of transparency are the prime factors. For diamond, it is determined by their brilliancy, optical purity and absence of color. For precious opal, it is determined by their play of color.
Rarity of gemstone is the most important part in the value of a gemstone, some degree of rarity must present in the gem. Glass, such as obsidian, may be transparent with depth of color, but is not rare. Rarity is also affected by supply and demand, as well as fashion trends and available supply of the material. Amethyst was once rare and expensive, until the discovery of the extensive Brazilian sources in the 18th century. There were also time that when violet was in fashion trend, demand for amethyst increase, then few year later, fashion trend changes and price decrease rapidly.
For gemstone to be suitable for jewelry it must be durable. A gemstone should be able to resist wear, abrasion such as dust, knocking, rubbing against other objects, tarnishing and chemical damage resulting from moisture or other atmospheric influences. A gemstone must be capable of lasting a long time without is beauty.
Portability of the gemstone has to be considered for its purposes. A gemstone for display can be big or large in size. However, when a gemstone is used for jewelry, it has to be cut to sizes that are suitable for the use in jewelry.