Inorganic Gemstones

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Rock Formation

Rocks: There are three major rock types: igneous, sedimentary and metamorphic.

Igneous Rocks

This rock type is formed by the solidification of molten magma either at the surface as volcanic lava, or within the Earth's crust.

Most of the common gemstones are formed in this rock type. Igneous rocks are divided into two (2) major types:

  1. Intrusive igneous rocks:
    Igneous rocks which solidify below the surface of the Earth are called intrusive rocks. Dykes, laccoliths, batholiths, stocks, pipes and pegmatite are various forms of intrusive rocks. Feldspar, quartz, beryl, tourmaline, topaz and zircon are some of the gemstones formed in pegmatitic rocks. Pegmatites generally represent the final phase of the crystallization of magma and are bodies associated with granitic rocks i.e. granular igneous rocks made up of quartz, feldspar and mica.
  2. Extrusive igneous rocks:
    Extrusive or volcanic rocks are formed when the magma erupted through the upper layers and crystallised by rapid cooling at the surface of the Earth. The fine grained texture of these rocks i.e. the relatively small size of the component mineral grains emphasizes the rapid rate of cooling of the magma. Slow cooling produces larger, well formed crystals. On the basis of their silica content, igneous rocks are divided into acid rocks intermediate, basic and ultra basic rocks.

Sedimentary Rocks

A rock exposed on the Earth's surface gets weathered (decomposed and disintegrated) into small particles. These are removed and carried away by the action of rain, flowing water (streams), moving ice (glaciers) and consolidates to form sedimentary rocks.

Some material is carried in solution and is precipitated when physical or chemical conditions change. These precipitates also form layers of sedimentary rocks. Sedimentary rocks are classified into three (3) types:

  1. Detrital sedimentary rocks:
    These are composed of fragments of rocks which were transported from the source by mechanical means, deposited and subsequently compacted, e.g. sandstone, conglomerate and shale.
  2. Chemical sedimentary rocks:
    These is formed when chemicals dissolved from the rocks are carried in solution to other areas where evaporation and precipitation of the dissolved materials takes place, e.g. limestone, opal and gypsum.
  3. Organic sedimentary rocks:
    These are formed by the accumulation of material of organic origin e.g. jet.

Metamorphic Rocks

This rock type is formed when igneous and sedimentary rocks in the various layers of the crust undergo changes as a result of a change in temperature and pressure conditions.

Typical metamorphic minerals are garnet, andalusite, kyanite, sillimanite, iolite, epidote etc. On the basis of the temperature, pressure and chemical conditions, metamorphic rocks are divided into various types.

  1. Contact or Thermal Metamorphism:
    Rock formation occurs at higher temperatures and low pressure, for example sandstone converted into quartz. Common contact metamorphic minerals are garnet, diopside, epidote and corundum.
  2. Regional or Dynamic Metamorphism:
    Rocks are formed due to rise in temperature and pressure. It involves larger rock masses. The main rock types are gneisses and schists which have a laminated banded appearance.

Gneiss is a coarse foliated rock with alternating discontinuous layer of minerals.
Schist is a finely foliated rock consisting of layers of different minerals which split into thin irregular plates.


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In Germany, the garland of ambers is hung around the necks of the infants, so that their teeth might come out without much trouble.

In Turkey, people fix a piece of amber in the hubble-bubble, dogged by an age old belief that the presence of the jewel would destroy all the germs and no disease would spread, even after many have dragged their puffs from it.

In Greece, people think if wine is poured into the cups made of amethyst, one does not get drunk after drinking that.

In Rome, a talisman having coral pieces is considered very auspicious to quell the bad spells of evil look. The Romans were in the habit of hanging chains made of ancient coral pieces from the cradle of the infants to protect them from evil effects.

In China, people wear rings studded with tiny conch or oyster shells as they believe it cures all aches such as stomach ache, worms and like.

In India, pendants made of silver and studded with pearls are hung from the necks of the children to save them from any bad effects.

Greeks still believe, if women wear blue sapphires then no sense of immorality would pollute their mind nor can any fear of the supernatural can trouble them.

The Pope Innocent III had circulated an order asking all the priests wear blue sapphire, so that morality can be strengthened.

It is said about turquoise, that when the stone changes its color into yellow, it actually signals bad times to the person who wears it.

It is believed that a turquoise gifted to a friend or a lover turns his or her life into one of happiness and good luck.

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