Rocks are the source of various chemical substances such as inorganic minerals or particular elements. However, these are also mined for the extraction of various precious and semi-precious jewels and diamonds.
Magma forms within the Earth’s crust and mantle. Heat melts the rock and makes it into molten magma. The composition of rocks, geological pressure, and water content influence the melting temperature of rocks also. When magma reaches the surface, it is termed as lava. The chemistry of the magma determines the chemical makeup of the rock that forms
Rocks (Study of rocks: petrology) and gemstones (the study of gemstones: gemology) are made of minerals (the study of minerals: mineralogy). Minerals are naturally occurring inorganic substances. Rocks don’t have a definite composition; however, each mineral possesses a characteristic shape and chemical makeup. Minerals are composed of one or more elements. Gold, silver, and Carbon are the only elements that form pure minerals. The normal table salt, well-known as rock salt, is also a mineral of sodium and chloride.
From thousands of known minerals, about 100 are the major constituents of rocks, and thus called rock-forming minerals.
Minerals crystallize when the molten lava or magma cools down within the earth or underwater (caverns). The precious of these are known as gemstones. Gemstones are professionally cut and polished to enhance their luster. These, when embellished in jewelry, increase their ornamental value. Non-crystalline pearls and amber are also considered gemstones.
About 4,000 inorganic minerals occur naturally, each characteristically distinct from others. Minerals are mined either for the extraction of specific elements or gemstones. Some gemstones are more precious than others. Precious gemstones include Rubi, Emerald, Sapphire, and Diamonds. The composition and appearance of these diamonds vary widely. Amethyst, Opal, and Turquoise are secondary to these precious gemstones. Amethyst was once considered a precious gemstone. Due to its abundant resources, it is now listed as a semi-precious gemstone. The value of all the gemstones is assessed by evaluating their physical features; such as durability, color, luster, cleavage, tenacity, fluorescence, streak, etc.
Different Types of Rocks
The high temperature and pressure under the earth’s crust and mantle produce gemstones. The gemstones we mine today were formed over millions of years from geological processes. These processes result in three types of rocks;
- Igneous rocks,
- Metamorphic rocks, and
- Sedimentary rocks
Most of the gemstones are Igneous in origin. Igneous rocks are formed from molten material including volcanic lava, ash as well as magma below Earth’s surface. These are either formed at depth within the earth’s crust at or near the surface from volcanic eruptions.
These can be Plutonic (intrusive) or Volcanic (extrusive) in nature. The temperature and the elements present play a role in determining the type and the texture of the rocks.
In the plutonic rocks, the magma cools down within the parent rock. The cooling is slow that forms larger, coarse crystals such as Granite.
The volcanic rocks form when molten lava erupts through a volcano or fissure. These are formed on/near the surface of the earth. The rapid cooling of the molten lava forms finely grained (Basalt), or glassy (Obsidian) rock structure.
At the surface, LAVA hardens to form volcanic rocks with tiny FINE-GRAINED crystals or GLASSY textures.
Beneath the surface, MAGMA hardens to form plutonic rocks with easily visible COARSE-GRAINED crystal texture.
TEXTURES OF ROCKS:
Igneous rock textures depend on cooling history.
- Fine-grained texture (Aphanitic) is due to fast cooling (at or near-surface).
- Coarse-grained texture (Phaneritic) is due to slow cooling at depth.
- Porphyritic texture characterize coarse crystals surrounded by fine-grained matrix (groundmass) forms due to initial slow cooling, then magma rising to (or close to) surface and the remaining magma cooling quickly
- The glassy texture is due to the very rapid cooling of magma cools such that crystals don’t have time to form. Obsidian (volcanic glass) forms this way.
- Vesicular texture is full of rounded holes (vesicles). It forms due to the escape of gas bubbles during the cooling of lava e.g. Pumice.
- Pyroclastic texture represents the chunks of molten material that fuse in extrusive rocks. This results from an explosive volcanic eruption that sends not only lava flying through the air but also fragments of the volcano itself.
Sedimentary Rocks are formed by the deposition and compaction of minerals. The term umbrella all the processes involved in the settling of organic and inorganic particles. Pebbles, sand, silt, clay, shells, and bones, all are sediments.
Metamorphic Rocks are formed when temperature or pressure changes the mineral composition of the preexisting rocks inside the Earth’s surface. Igneous, sedimentary, or even metamorphic rocks can be transferred to metamorphic rock.
Gemstones are not as abundant as individual minerals. And a rock doesn’t need to be found where it is formed. Plate tectonics and volcanic eruptions displace the rocks to about 400km to the surface, subsequently the gemstones. The gemstones found in their original place are called primary deposits. Many primary deposits are present among pegmatite veins.
With the cooling of the magma and crystallization of most minerals, residual water gets concentrated with the un-crystallized minerals. This is a concentrated solution is Pegmatite. The gemstone that’ll crystallize depends on its concentration in the parent rock. Tourmaline, Beryl, Topaz, Quartz, Feldspar, Zircon, Graphite, and Apatite are the Pegmatite gems.
If the primary deposits get eroded or weathered, the gemstones are also displaced with water. These are compacted and deposited along river or delta as gem pockets or secondary deposits.
Which Rocks have Gemstones?
Igneous-origin gemstones include Amethyst, Beryl, Citrine, Feldspar, Moonstone, Apatite, Topaz, Tourmaline, Asbestos, Peridot, and Quartz to name a few. These are found in the composition of various minerals. However, most of them contain silicate, a mineral. Basalt contains semi-precious gemstones such as Zircon, Corundum, and Garnets. These are low in silica.
Glassy Obsidian rocks are polished into gemstones. There are various varieties of obsidian including Rainbow Obsidian, Red Mahogany Obsidian, Silver Sheen Obsidian, Pumpkin obsidian, and Apache tree obsidian.
Of thousands of the minerals discovered so far, a few are famous. beryl, topaz, tourmaline, quartz, spinel, turquoise, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine, opal, and zircon. These provide more than one gemstone.
Or you can say it as some gemstones have common mineral elements. Rubies and Sapphires are made of conundrum (Aluminum oxide) minerals. Beryl (beryllium, aluminum, silicon, oxygen) forms an Emerald and Aquamarine. Their colors come from trace metals such as chromium, titanium, etc.
Diamond is also igneous in origin; formed in Mantle under extreme temperature and pressure conditions. All gemstones are composed of two or more minerals, except diamond. It consists of pressurized carbon only. For this reason, it is the hardest substance on Earth. Igneous rock called Kimberlite is the source of the most sought-after gemstone.
Sedimentary rocks are home to gemstones like Zircon, Opal, and Jasper. Gold, Emerald, Jade, Ruby, and Zircon would be found in the metamorphic rocks.
Gemstones as Jewels
From purple colored Amethyst to pale blue Aquamarine, gorgeous green Emerald to yellow-green Peridot, from hardest Diamonds to precious Pearls; all of the gemstones are an important part of the jewelry.
Gemstones are cut and polished for the very purpose. Luster, color or transparency, and durability are the prime requisite of the gemstones as jewelry. The durability of the gemstones is compared with the Moh’s scale. The hardest gemstones as Diamonds, Conundrum, and Topaz have the highest values i.e. 8 to 10. While Calcite, Gypsum, and Talc, have the lowest values on the scale.
Considering the hardness of the jewels is important. The gemstone has to be durable to withstand wear and tear over the years.
The cutting of gemstones is very important. It plays role in giving the gemstone its brilliant luster. For this purpose, sound comprehension of the crystal structure is required. Until the discovery of the abrasive property of diamonds in the 15th century, the Cabochon cut was preferred. Various methods of faceting diamonds have been developed so far like rose cut, brilliant cut, step cut, and drop cut. The color of the gemstones is also enhanced nowadays. These increase the value of the gemstone greatly. Pairing the gemstones with precious metals add to their charm and value.
The gemstones are not a rare commodity in present days. The gemstone-studded rings and necklaces and other accessories are quite trendy. Natural gemstones bear exorbious costs. However, some gemstones like ruby, sapphire, and emeralds are now artificially synthesized.