INTRODUCTION
Gemstones are a type of mineral crystal which, in cut and polished form, can be used to make jewelry or other ornaments. However, certain rocks (such as lapis lazuli and opal) and occasionally organic materials that are not minerals (such as amber, jet, and pearl) are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity and notoriety are other characteristics that impart value to gemstones. A gemstone is also known as a jewel, precious stone, semiprecious stone etc.
Characteristics and classification
In modern use, the precious stones are Ruby, Emerald, Sapphire and Diamond, with all other gemstones being semi-precious. This distinction reflects the rarity of the respective stones as well as their quality. Except for the colorless diamond, all are translucent with fine color in their purest forms with hardness of 8 to 10 on the Mohs scale. Other stones are also classified by their color, translucency, and hardness. The traditional distinction does not necessarily reflect modern values; for example, while garnets are relatively inexpensive, a green garnet called tsavorite can be far more valuable than a mid-quality emerald. Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone. Use of the terms ‘precious’ and ‘semi-precious’ in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not necessarily the case.
In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminium oxide.
3). Many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form in which the gem is usually found in. For example, diamonds, which have a cubic crystal system, are often found as octahedrons.
Gemstones are classified into different groups, species, and variety. For example, ruby is the red variety of the corundum species, while any other color of corundum is considered sapphire. Other examples are the emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink), which are all varieties of the mineral species beryl.
Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fractures and lustre. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum. Material or flaws present within a stone are known as inclusions. Gemstones may also be classified in terms of their “water”. This is a recognized grading of the gem’s luster, transparency, or “brilliance”. Very transparent gems are considered “first water“, while “second” or “third water” gems are those of a lesser transparency.
Gemstone pricing and value are governed by factors and characteristics in the quality of the stone. These characteristics include clarity, rarity, freedom from defects, the beauty of the stone, as well as the demand for such stones. There are different pricing determiners for both colored gemstones, and for diamonds. The pricing on colored stones is determined by market supply-and-demand, but diamonds are more intricate. Diamond value can change based on location, time, and on the evaluations of diamond vendors.
TABLE OF CONTENTS
Agates
Agate is the banded form of the mineral Chalcedony, which is a microcrystalline variety of Quartz. Agate is the most varied and popular type of Chalcedony, having many varieties on its own. … The formation of Agate is most often from deposition of layers of silica filling voids in volcanic vesicles or other cavities.
Color: Green, Blue, Black, Red, White, Yellow, Purple, Pink, Grey, Brown, Orange
Lustre: Vitreous
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Hardness (Mohs hardness scale): 7
Mineral class: Chalcedony
Alexandrite
Often described by gem aficionados as “emerald by day, ruby by night,” alexandrite is the very rare color-change variety of the mineral chrysoberyl. Originally discovered in Russia’s Ural Mountains in the 1830s, it’s now found in Sri Lanka, East Africa, and Brazil, but fine material is exceptionally rare and valuable.
Color: Green, Blue, Yellow, Red, Pink, Purple, Grey
Lustre: Vitreous
Birthstone zodiac sign: Gemini
Associated month: June
Crystal system: Orthorhombic crystal system
Chemical formula: BeAl₂O₄
Hardness (Mohs hardness scale): 8.5
Amber
Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects. Amber is used in jewelry. It has also been used as a healing agent in folk medicine.
Color: Green, Blue, Red, Yellow, Brown, Orange
Lustre: Resinous
Crystal system: Amorphous solid
Hardness (Mohs hardness scale): 2 – 2.5
Transparency: Opaque, Transparent
Refractive index: 1.539 – 1.545
Amethyst
Amethyst is a well known mineral and gemstone. It is the purple variety of the mineral Quartz, and its most valuable and prized variety. … Some varieties may also change to a light green color, which is given the trade name “Prasiolite”, or “Green Amethyst”, as it is more commonly known in the gem trade.
Color: Purple
Lustre: Vitreous
Birthstone zodiac sign: Aquarius
Associated month: February
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Aquamarine
Aquamarine is the name used for the mineral beryl within the colour range of greenish blue to blue. … Aquamarine gemstones can range from a very light hue to much deeper, vibrant shades of blue. The term aquamarine is derived from a combination of two Latin words.
Color: Blue
Lustre: Vitreous
Birthstone zodiac sign: Pisces
Associated month: March
Crystal system: Hexagonal crystal system
Chemical formula: Be₃Al₂SiO₆
Hardness (Mohs hardness scale): 7.5 – 8
Blood stone
Bloodstone is a beautiful green stone that features red or brown spots that form as a result of impurities in the iron oxide. These spots look like blood splatters, giving the stone its English name.
Color: Green, Red, Brown
Lustre: Vitreous
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Hardness (Mohs hardness scale): 7
Mineral class: Chalcedony
Transparency: Opaque
Cat’s Eye
Chatoyance is an optical phenomenon in which a band of reflected light, known as a “cat’s-eye,” moves just beneath the surface of a cabochon-cut gemstone. … Excellent specimens of chrysoberyl exhibit the finest chatoyance, and tiger’s-eye is the chatoyant gem most widely used in jewelry.
Color: Blue, Green, Red, White, Black, Yellow, Purple, Pink, Grey, Brown, Orange
Lustre: Vitreous, Waxy
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Hardness (Mohs hardness scale): 6.5 – 7
Transparency: Opaque, Translucent
Citrine
Citrine is the yellow to brownish-red variety of the mineral Quartz. It is a widely used as a gemstone, and after Amethyst it is the most popular Quartz gem. Most Citrine is formed by heat treating purple Amethyst. … Natural light yellow Citrine is often called “Lemon Quartz” on the gemstone market.
Color: Yellow, Orange, Brown
Lustre: Vitreous
Associated month: November
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Hardness (Mohs hardness scale): 7
Mineral class: Quartz
Coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.
Unlike most other gemstones which are of mineral origin, Coral is organic, formed by living organisms. It forms from branching, antler-like structures created from coral polyps in tropical and subtropical ocean waters. … This Red Coral, or Precious Coral as it is often known by, is the most used gemstone form of Coral.
Coral gem’s chemical composition is also important for the formation of these sea animals. The basic composition of this gem is calcium carbonate (CaCO3). The specific gravity is 2.65 and the refractive index ranges between 1.486 and 1.658. Its hardness on the Mohs scale is 3.5 to 4.
Emerald
Emerald is a variety of the beryl species and is related to aquamarine and green beryl. The first known emerald mines were in Egypt. … Emerald has a hardness of 7.5-8.0 on the Mohs Hardness Scale. Emerald is the only stone with a cut named after it.
Color: Green
Lustre: Vitreous
Birthstone zodiac sign: Taurus
Associated month: May
Crystal system: Hexagonal crystal system
Chemical formula: Be₃Al₂SiO₆
Ivory
Ivory is a hard, white material from the tusks (traditionally from elephants) and teeth of animals, that consists mainly of dentine, one of the physical structures of teeth and tusks. The chemical structure of the teeth and tusks of mammals is the same, regardless of the species of origin.
It has no intrinsic value, but its cultural uses make ivory highly prized. In Africa, it has been a status symbol for millennia because it comes from elephants, a highly respected animal, and because it is fairly easy to carve into works of art.
Jade
Jade, either of two tough, compact, typically green gemstones that take a high polish. Both minerals have been carved into jewelry, ornaments, small sculptures, and utilitarian objects from earliest recorded times. The more highly prized of the two jadestones is jadeite; the other is nephrite.
Color: Green, Black, White, Red, Blue, Yellow, Purple, Pink, Grey, Orange, Brown
Lustre: Vitreous, Waxy
Crystal system: Monoclinic crystal system
Chemical formula: NaAlSi₂O₆
Hardness (Mohs hardness scale): 6 – 7
Mineral class: Jadeite
Transparency: Opaque, Translucent
Moonstone
Moonstone is a mineral of the orthoclase feldspar group and is composed of potassium aluminum silicate. It is distinguished from other similar stones by the presence of adularescence. … Plagioclase feldspar is composed of calcium and sodium and orthoclase feldspar is composed of potassium.
Color: Blue, Green, White, Yellow, Pink, Purple, Grey, Orange, Brown
Lustre: Pearly
Chemical formula: (Na,Ca)Al₁₋₂Si₃₋₂O₈, KAlSi₃O₈
Hardness (Mohs hardness scale): 6 – 6.5
Mineral class: Orthoclase, Oligoclase
Transparency: Translucent, Transparent
Refractive index: 1.518 – 1.526
Opal
Opals can have fiery play-of-color, but they can also be colorless, opaque, translucent or transparent. Opal has a hardness of 5.0-6.5 on the Mohs Hardness Scale. Due to their relative softness, opals are rarely faceted. … Opals are mainly cut as cabochons or fashioned as beads.
Color: Blue, Black, Green, White, Yellow, Pink, Red, Purple, Grey, Brown, Orange
Lustre: Pearly, Vitreous, Waxy
Birthstone zodiac sign: Libra
Associated month: October
Crystal system: Amorphous solid
Chemical formula: SiO₂·nH₂O
Hardness (Mohs hardness scale): 5.5 – 6.5
Periodt
Peridot is a well-known and ancient gemstone, with jewelry pieces dating all the way back to the Pharaohs in Egypt. The gem variety of the mineral Olivine, it makes a lovely light green to olive-green gemstone. … The most desirable color of Peridot is deep olive-green with a slight yellowish tint.
Color: Green, Yellow
Lustre: Vitreous
Birthstone zodiac sign: Leo
Associated month: August
Crystal system: Orthorhombic crystal system
Chemical formula: (Mg,Fe)₂SiO₄
Hardness (Mohs hardness scale): 6.5 – 7
Rose Quartz
Rose quartz, translucent, usually turbid, very coarse-grained variety of the silica mineral quartz found in pegmatites. Rose quartz is valued for its pale- to rich-pink colour, which may be due to titanium. It has been carved since early times and has been faceted to provide gems of good brilliance.
Color: Pink
Lustre: Vitreous
Crystal system: Hexagonal crystal system
Chemical formula: SiO₂
Hardness (Mohs hardness scale): 7
Mineral class: Quartz
Transparency: Translucent, Transparent
Ruby
Ruby is a variety of the mineral species corundum and is related to sapphire, another variety of corundum. … Ruby is a durable gemstone with a hardness of 9.0 on the Mohs Hardness Scale. Ruby gemstones are cut into round, pear, oval, cushion, emerald-cut and other shapes.
Color: Red
Lustre: Adamantine, Vitreous
Birthstone zodiac sign: Cancer
Associated month: July
Crystal system: Hexagonal crystal system
Chemical formula: Al₂O₃
Hardness (Mohs hardness scale): 9
Sapphire
Sapphire is the most precious and valuable blue gemstone. It is a very desirable gemstone due to its excellent color, hardness, durability, and Lustre. In the gem trade, Sapphire without any color prefix refers to the blue variety of the mineral Corundum.
Color: Blue, Green, White, Yellow, Black, Pink, Purple, Grey, Orange, Brown
Lustre: Adamantine, Vitreous
Birthstone zodiac sign: Virgo
Associated month: September
Crystal system: Hexagonal crystal system
Chemical formula: Al₂O₃
Hardness (Mohs hardness scale): 9
Tanzanit
Tanzanite is a variety of the mineral species zoisite. … Tanzanite has a hardness of 6.0-7.0 on the Mohs Hardness Scale. Some tanzanite gemstones are carved or sculpted by famous lapidary artists. It is only found in one place on earth, Tanzania.
Color: Blue, Purple
Lustre: Vitreous
Associated month: December
Crystal system: Orthorhombic crystal system
Chemical formula: Ca₂Al₃(SiO₄)₃(OH)
Hardness (Mohs hardness scale): 6 – 6.5
Mineral class: Zoisite
Turquoise
Turquoise is a sky-blue or green translucent to opaque basic aluminum phosphate that contains copper. It has been prized as a gemstone for millennia. … The color that we recognize as turquoise was named after this gemstone. Turquoise was used by the ancient Egyptians and Aztecs as a jewelry gemstone and decorative stone.
Color: Blue, Green
Lustre: Waxy
Birthstone zodiac sign: Sagittarius
Associated month: December
Crystal system: Triclinic crystal system
Chemical formula: CuAl₆(PO₄)₄(OH)₈ • 4H₂O
Hardness (Mohs hardness scale): 5 – 6