Gemstones and Birthstones

How Gemstones are Formed

This Earth started as a lump of hot gas (the same way as the stars, sun and planets) with temperature of millions of degrees Fahrenheit, and then it liquefied, froze from the outside and formed a hard outer shell (crust). It is on this stone shell or crust that living things existed: simple forms of animal (600 million years ago), land plants (475 million years ago), amphibians and reptiles (about 300 – 400 million years ago), mammals (200 million years ago), birds and flowers (between 100 – 200 years ago) and finally human (about 200,000 years ago, and now is growing in population of billions).

The age of Earth is estimated to be between 4 – 5 billion years old. Inside the earth there is a hot liquid core of molten rock, and on the outside you will find a hard crust. The crust on the earth is just like the shell on an egg (if you compare the earth to an egg). The crust is made up of rocks and minerals. Most gemstones are found in this crust of the earth (the earth’s top layer) with a depth of 3 – 25 miles (5 – 40 km). The earth’s crystal were mostly formed millions of years ago. Gemstones are often what people mean when they are talking about “crystals”.

Below the earth’s crust there is another layer which is the mantle. Peridot and diamond are one of a few gemstones that form in the mantle of the earth, which represents 83% of the earth’s volume. The earth’s mantle consists mostly of magma or melted rock (3000 km thick) with a solid upper layer.

All gemstones originally mined in the crust. The crust itself is made up of 3 kinds of rocks: igneous, metamorphic and sedimentary. Those technical terms refer to the way in which rocks formed. Certain gemstones are associated with one type of rock while others with multiple kinds.

Igneous rocks are rocks which have solidified from the molten rocks. The igneous process involves the solidificaton of magma (igneous means made from heat or fire). Through the volcanic pipe, the magma from the mantle rise to the crust and if it reaches the earth’s surface, then the liquid is called lava, and it will solidify into igneous rocks. When the lava cools too quickly on the surface, then obsidian is created. But if the magmatic mass slowly cools in the crust, then it crystallize to form minerals. When the pressure is increased, this pegmatitic fluid can infiltrate surrounding rocks and make chemical exchanges with them.

The rate of solidification determines the size of the crystals. Slower the rate, larger the crystal and of course larger will be the size of the gemstones in it. Gemstones that are formed from igneous rock include the beryls (aquamarine, morganite and emerald), all of the quartzes (amethyst, ametrine, amethyst etc), the chrysoberyl group, the garnets, apatite, spinel, tourmaline, zircon, moonstone, diamond, tanzanite and topaz.
Sedimentary rocks are rocks formed by accumulation of rock fragments produced by weathering. Once the igneous rock reaches the earth’s surface, the forces of erosion and weathering produce smaler particles which accumulate on the surface or are moved by water and wind. As time passes, layers of these sediments build up either on land or under water. Sedimentary rock is created when the pressure from upper layers causes compaction in lower layers along with variuos chemical and physical changes such as lithification. Another process which produces sedimentary rocks is evaporation, as when dripping mineral-laden waters leave behind stalagmites or stalactites. Sedimentary rocks cover about 75% of the earth’s surface. Some gemstones that associated with sedimentary rock include malachite, jasper, zircon and opal.
Metamorphic rocks are rocks that have changed. They are either sedimentary or igneous which have change beneath the earth due to pressure and heat and has resulted in new form of minerals in it. The presence of intrusive magma in an area (contact metamorphism), or of tectonic plate interactions on larger scale (regional metamorphism) puts the sedimentary and igneous rocks and minerals under pressure or heat which may cause changes in their crystal structure and chemistry. The result is the creation of metamorphic rocks. Metamorphic rocks are the least commonn of the 3 types of rocks. Beryl, jade, turquoise, zircon, sapphire, ruby, spinel and lapis lazuli are the gemstones associated with metamorphic rock.

The Rock Cycle is the minerals and rocks that are in a constant state of change. Igneous rock can change into metamorphic or sedimentary rock. Sedimentary rock can change into igneous or metamorphic rock. And metamorphic rock can change into sedimentary or igneous rock. The rocks are constantly being formed, worn down and form again, and it takes about millions of years for them to change. It is just like water cycle but it takes a lot longer.

Mohs Scale of Hardness

Gemstones

Hardness

Diamonds
Corundum; Rubies and sapphires
Chrysoberyl, Alexandriet, Cat’s Eye
Spinel and Topaz
Beryl, Aquamarine, Emerald, Garnet, Tourmaline, Zircon
Agate, Amethyst, Chalcedony, Chrysoprase, Demantoid, Heliotroop, Jade, Onyx, Sardonyx, Smoky Quartz, Tiger's Eye
Jadeite, Peridot, Sardonyx,
Lapis Lazuli, Opal, nephrite, turquoise, Obsidian
Malachite

10
9
8.5
8
7.5

6.5
5.5-6
4

Precious stones are highly sought after and are favourites among human, not only for its accesory nature but also because they are believed to contain hidden powers. Before they are used as sacred items or jewellery, the stones are usually polished to make it shine. Gemstones or jewellery lovers are growing in number day by day.