All the gemstones have some amount of latent electricity which can be experienced in different ways. This latent form of electricity is basically of static in nature and an be produced in three different ways:

1) Friction
2) Heat
3) Pressure

1) Electricity produced by friction or Frictional Electricity: When a gemstone is rubbed with a cotton or woolen cloth, some electric charge is produced in the gem. In some stones the charge is so intense that it can even burn the cloth that causes it to appear. Diamond is such a stone that produces intense electric charge on rubbing. Besides diamond, topaz, Shobha Mani, Trinakant Mani etc are some other jewels that produce intense electric charge.

2) Thermal Electricity: Heating a gemstone also produces some electric charge in it. But there is a threshold limit below which heating does not produce any charge in the stone. Rock crystal is the prominent gemstone that produces intense charge upon heating.

3) Piezo Electricity or charge due to pressure: If a certain amount of pressure is applied on a crystal, it begins to act like a battery cell and produces electric charge. With the removal of pressure, the crystal regains its original state. Rock crystal is again an efficient gemstone to produce electric charge under pressure.

Alexandrite: It is an extremely beautiful gem and like Opal it is also very sensitive to light. It glows like a rainbow of colours. Hence it is also known as colour changing stone. It is found in Sri Lanka and Africa. Mostly, Alexandrites with deep indigo colour are available with the jewellers. It is regarded as a potential substitute for blue sapphire.

Katiala: It belongs to crystal category and is found in deep and light voilet colours. In common parlance and the jewellers terms it is called as Jamuniya (amethyst). It is used as a substitute for blue sapphire. It is mostly found in Hampshire (England), Germany, Iran, Balkan countries, Madagascar, Mexico and some parts of India. It also changes its colour when heated in fire.

Kaharava: It is mostly found in brown colour. It is opaque like amber. Kaharava is also a fossilized resin of pine that has metamorphosed into stone. It may be found in many colours like yellow, blue, white, red and green. It has a sweet, camphor-like smell. Due to its property to attract grass, paper etc. when charged, Kaharava is also known as Trinakant Mani or Trinakarsh. It is mostly found in Thailand, Romania, Myanmar, Brazil and some parts of India. Madagascar and Peru are the regions where Kaharava is found aplenty. But it has more medicinal uses than astrological.

Kasoti (Touch stone): Jewellers use this stone to test the purity of gold. It is mostly used in making crockery, utensils and mortar etc.

Calamine: Some crafty jewellers sell it as a substitute for blue sapphire but in fact it cannot replace blue sapphire. It is known as black star and presents a complete rainbow of colours in flood light.

Kurund: This stone is found in many colours like violet, brown, blue, white, green, red, black and pink. It is mostly used to make the grinding stone that the jewellers use to scourge gemstones. It is mostly found in Thailand, lower regions of Russia, Myanmar, Srilanka, Canada, Australia and India.

Kakarnili: Due to its deep blue colour, many astrologers prescribe it as a substitute for blue sapphire. It is indeed effective but not as much as blue sapphire. But there is no guarantee that you get genuine Kakarnili in the market because most jewellers sell synthetic stone instead. Kakarnili is mostly found in Kashmir.

Kudaratta: It is black stone that has mote decorative uses than astrological ones. It is cheaper and appears attractive due to yellow-black spots on it.

Khara: It is an ordinary stone available in black colour. It radiates green light. It is also mostly used in medicines.

Agate: It belongs to the Navaratna category of the gemstones. It is mostly used to please the shadow planet Rahu. It is known as Gomed in Hindi, Gomed Mani, Gomedak, Tapo Mani, Peet Mani, Peet Sphatic in Sanskrit. In Arabian and Persian it is known as Zargun, Hazarya Mani, Siduri etc. In Marathi it is known as 'Gomed Mani'. In colloquial terms, it is called zircon, but it is a misnomer. Even in common English its often referred to as Zircon. But the correct term for it is Hassonite or Agate.

A transparent Agate is regarded as the best. But it is available in many colours: black, reddish brown, red, orange and occasionally green. Despite being cheap, it is an extremely effective gemstone. It is found in the Orissa province of India besides Myanmar, China, Arab, Florida, South Africa and Sri Lanka. Best Agates come from Sri Lanka. Agate is prescribed by the astrologers for the people to get rid of many kinds of problems. It has profound effect on the diseases also. But one should imperatively get his horoscope scrutinized by an able astrologer.

Moonstone: It is an inexpensive stone. It resembles with the teeth of a cow in appearance. It is prescribed as a substitute for pearl. It is found in small bits in Myanmar, India, North America and Sri Lanka. It is also found in Brazil and Switzerland.

Gauri: It is brown or green coloured stone that belongs to Akeek category. Streaks of many colours are present on it. Hence it is also known as rainbow stone. It gives mental peace besides riches and repute. It can be worn on any auspicious day.

Gudari: It is a cheap, ordinary stone mostly worn by the ascetics, Muslims etc. specially wear big Gudari. It is found in red, yellow and slaty colours.

Great Indian mathematician and astrologer of the yore, Varahmihir has written profusely about the gemstones in his famous treatize Vrihatsamhita.

Following is the names of the gemstones described in Vrihatsamhita.
Vajramani (Heera) Diamond. Padma r Vimalak. Marakat (Panna) Emerald. Vaidurya (Lahsunia) Cat's eye. Sphatic (Rock Crystal) Sangandhik. Indraneel Mani (Neelam) Blue Sapphire. Rudhir. Raja Mani. Gomed (Gomedak) Hassonite.	Pushprag (Pukraj) Yellow Sapphire. Mukta (Moti) Pearl. Samyak. Shankh. Karkotak. Pulak. Shashikant. Mahaneel. Jyotiras. Brahmin Mani. Praval (Munga) Coral.

Thus, only twenty-two gemstones have been described in Vrihatsamhita. You may ask: Why? It can only be said about it that during the period of Varahmihir only these twenty-two gemstones could have been in fashion.

OPAQUE GEMSTONES

Many elemental impurities that are present in the crystal lattice of a gemstone not only increases its density but also makes it opaque to light. Though presence of these impurities does not affect the utility of the stone. Akik and Phiroza (turquoise) are the famous example of opaque gemstones. These two stones are regarded particularly miraculous.

TRANSLUCENT GEMSTONES

These fall between the transparent and opaque categories. Many important gemstones come under the category of translucent stones. These stones are not as clear as the transparent stones nor as filthy as the opaque ones. Hence, the translucent gemstones can aptly be described as dim gemstones.

Regardless of the category, gemstones have more importance than the ordinary stones and are hence more in demand despite their high value. To be valuable a gemstone should fulfill some conditions like smoothness, better cut, brilliance etc. Apart from these hardness is another condition that a gemstone should comply to be designated as a jewel. Gemstones are invariably harder than the ordinary stones.

HARDNESS OF THE GEMSTONES

Composition of the chemical compounds, their mixture, relative density, crystal lattice etc. together determine the hardness of a gemstone. Since the chemical composition varies from stones to stones hence each and every gemstone has its unique hardness which commonly does not match with that of the other. Hardness is the main property of a gemstone. Diamond is the hardest gemstone. In fact, it is the hardest substance in the world. Besides it, emerald, blue and yellow sapphires and ruby are also hard. Coral is also hard but interestingly it is not a true stone. It is in fact a fossil and organic in origin.

Next to hardness it is specific density that differentiates one gemstone from another.