More than 200 gemstones occur in nature, most of them being natural minerals. A mineral needs to have qualities such as hardness, toughness, clarity and rarity to be considered as a gemstone. Many gemstones possess a color on account of the minerals or even impurities present in them. Copper, chromium, iron, manganese and titanium are some minerals which give gemstones their characteristic colors. On the other hand, very small amount of impurities can lead to a change in color as it can affect the atomic structure of the gemstone. For instance, in its pure form, corundum (aluminum oxide) is colorless, but it can turn red in the presence of chromium (as in ruby) or an intense blue if it contains titanium (as in sapphire). In this manner, trace impurities can result in the formation of various colored gemstones. However, all gemstones are not minerals. While crystalline gemstones like ruby, emerald and garnet are minerals, with a definite atomic structure, amorphous gemstones like amber and opal are not minerals as they have no definite internal atomic structure.

Characteristics of Mineral Gemstones:

The two main characteristics of mineral gemstones are specific gravity and hardness.

Specific gravity refers to the ratio of density of the gemstone to the density of water. In other words, it measures the number of times a volume of the gemstone is heavier than an equal volume of water.

Hardness is the resistance of a gemstone to scratching. It is measured on the Mohs scale, which is a standard scale of 10 minerals.

Based on these two attributes, all mineral gemstones are classified into 16 groups in all. These are listed as follows:

1. Beryl: This group of gemstones is chemically called beryllium aluminum silicate and has a hardness of 7.5-8 on Mohs scale and a specific gravity of 2.63-2.91.

Emerald, aquamarine, red beryl and goshenite fall under this category.

2. Chrysoberyl: Chemically termed as Beryllium aluminum oxide, it measures an 8.5 on Mohs and has a specific gravity of 3.68-3.78. The mineral gemstones in this group include cats eye, chrysoberyl and alexandrite.

3. Corundum: The chemical name for this category is aluminum oxide and its hardness on Mohs scale is 9 while the specific gravity is 3.96-4.05. The deep red ruby and blue sapphire are varieties of corundum.

4. Diamond: Known to be the hardest known substance, diamond is chemically carbon and it stands at 10 on Mohs and has a specific gravity of 3.51. It is colorless to faint yellow, depending upon the quality.

5. Feldspar: This category consists of two distinct minerals-the alkali feldspar and the plagioclase, with a hardness of 6-6.5 on Mohs and a specific gravity of 2.55-2.76. The former category has gemstones like orthoclase, moonstone and amazonite while the latter has sunstone and peristerite.

6. Garnet: Chemically, this gemstone group constitutes of silicate minerals, with a hardness of 6.5-7.5 Mohs and 3.5-4.3 as specific gravity. Some of the gemstones in this group are almandine, andradite, topazolite, hessonite and pyrope.

7. Jade: This group contains the gemstones nephrite (calcium magnesium silicate) and jadeite (sodium aluminum silicate), with hardness 6 on Mohs and specific gravity as 2.9-3.1 and 3.1-3.5 respectively.

8. Lapis Lazuli: This group is made up of lazurite, with some amounts of pyrite and calcite. The gemstone measures 5-5.5 on Mohs and has a specific gravity of 2.7-2.9.

9. Opal: This gemstone group is hydrated silica, with hardness 5.5-6.5 on Mohs and a specific gravity equal to 1.98-2.25. White opal, black opal, water opal and fire opal are the gemstones included in this group.

10. Peridot: Chemically, this olive to lime green gemstone is composed of magnesium iron silicate, which has a hardness of 7 Mohs and a specific gravity of 3.22-3.45.

11. Quartz: This gemstone group has a chemical composition of silica, hardness 7 Mohs and specific gravity of 2.65. Amethyst, citrine, morion, rose quartz and green quartz come in this group.

12. Spinel: Magnesium aluminum oxide is the chemical composition of this group of gemstones, which has a hardness of 8 Mohs and specific gravity of 3.58-4.06. Rubicelle, sapphire spinel and chlorspinel are some stones in this group.

13. Topaz: This group of wine yellow, red, violet, pale blue or green gemstones is chemically made of aluminum silicate fluoride hydroxide and measures 8 Mohs and has a specific gravity of 3.5-3.6.

14. Tourmaline: Chemically called complex aluminum borosilicate, this category of gemstones has a hardness of 7-7.5 Mohs and specific gravity of 3.03-3.25. The gemstones in this group include anchorite, rubelite, Brazilian emerald and siberite.

15. Turquoise: The greenish blue gemstones have the chemical name hydrous copper aluminum phosphate, have a hardness of 5-6 Mohs and specific gravity of 2.6-2.8.

16. Zircon: Chemically termed as zirconium silicate, this group of mineral gemstones has a hardness of 7.5 Mohs and specific gravity of 4.6-4.7. Hyacinth and matura diamond fall under this category.

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