Hardness and specific gravity are two of the major characteristics of gemstones.

Hardness of a gemstone is its resistance to scratching and may be described relative to a standard scale of 10 minerals known as the Mohs scale. F. Mohs, an Austrian mineralogist, developed this scale in 1822.

According to Mohs scale, the hardness of--
Talc is 1; Gypsum is 2; Calcite is 3; Fluorite is 4; Apatite is 5; Feldspar is 6; Quartz is 7; Topaz is 8; Sapphire is 9 and Diamond is 10

Specific gravity is the number of times heavier a gemstone of any volume is than an equal volume of water; in other words, it is the ratio of the density of the gemstone to the density of water.

The 16 mineral gemstone groups listed below are highly prized for their beauty, durability, and rarity: 

Beryl
It has hardness of 7.5 - 8 Mohs. It is formed of beryllium aluminum silicate and its specific gravity is 2.63-2.91.

Colour appeals to our feelings directly. It makes us happy and cheerful, livens us up or calms us down, and has a magical or liberating effect. And where is colour more lastingly and more beautifully captured than in a gemstone? In the fascinating world of precious stones, emeralds glow in the fieriest green imaginable. Aquamarines sparkle in a whole range of blues - from the light blue of the sky to the deep blue of the sea. And the charming pink of morganite puts a spell on women the whole world over. Yet how many people are aware of the fact that these gems, different as they are, belong to a single family? Aquamarine, emerald, and morganite are all beryls - just like golden beryl, yellowish-green heliodor, colorless goshenite and the rare red beryl. Whether blue, green, yellow, colourless or pink, their chemical and physical properties essentially correspond; it is only in their colours that they differ from one another so much.

The following varieties of gemstones come under it -
Emerald: Intense green or bluish green
Aquamarine: Greenish blue or light blue
Morganite: Pink, purple pink, or peach
Heliodore: Golden yellow to golden green
Red beryl: Raspberry red
Goshenite: Colorless, greenish yellow, yellow green, brownish

Chrysoberyl
It has hardness of 8.5 Mohs. It is formed of beryllium aluminum oxide and its specific gravity is 3.68 - 3.78.

Like the eye of a panther, cats-eye chrysoberyl seems almost supernatural in origin. How could something so feline be mineral and not animal? Cats-eye chrysoberyl, a cousin of color-changing alexandrite, is a variety of chrysoberyl, which has a distinct band of light across its face, which sweeps from side to side.

The color ranges from a honey-brown to an apple green with rich gold colors generally the most valued. The most important value factor is the strength and sharpness of the eye. Fine cats-eye chrysoberyl often also shows the "milk and honey" effect. When a bright light source is directed at the side of the stone, one side of the eye will be milky white and the other remains gold. When the stone is rotated, the colors switch. Cats-eyes are especially popular in mens jewelry.

Chrysoberyl can also seen in a faceted variety, which has a honey-gold color. It may remind you of yellow sapphire, topaz, or citrine.

The following varieties of gemstones come under it -
Chrysoberyl: transparent yellowish green to greenish yellow and pale brown
Alexandrite: red in incandescent light and green in daylight
Cats eye: usually yellowish or greenish

Corundum
It has hardness of 9 Mohs. It is formed of aluminum oxide and its specific gravity is
3.96 - 4.05. The following varieties of gemstones come under it -
Ruby: Intense red
Sapphire: Blue

Diamond
It has hardness of 10 Mohs. It is formed of carbon and its specific gravity is 3.51. 

Unique in the world of gemstones, Diamond is the hardest of all materials. Perhaps it is because of this durability that diamonds are treasured as symbols of devotion and purity.