AJS Gems Limited. Fine gemstones from Bangkok, Thailand-Gemstone Education

The difference between an ordinary stone and a gemstone is beauty, durability, rarity and the way in which it reflects and refracts light. Most loose gemstone are minerals that have been shaped as rough stones or flawed crystals within rocks below the earth's surface. They have a specific chemical composition and atomic structure arranged in ordered patterns that produce the physical and optical properties used to define and identify them.

Skillful cutting and polishing is needed to reveal the full extent of a gemstone's beauty. When a lapidary cuts and polishes a gemstone it also tries to maintain as much of its weight as possible. It must also carefully consider the stone's hardness, dispersion, birefringence, cleavage, flaws and inclusions. The cut used depends on whether the gemstone is colored, transparent, opaque, or translucent and whether it has strong iridescence or sheen and whether it is a stone with a cat's eye or star.

A cabochon cut results in a rounded or curved outline and a smooth,polished surface. It is frequently used for gemstone that are either opaque, translucent, strongly colored or have a star or cat's eye.

The brilliant cut produces many facets -- flat, polished faces -- and is frequently used for transparent gemstone such as diamonds because it optimizes dispersion and luster. The step or trap cut is used for gems such as rubies where color is the most important feature.

The luster of a gem is the amount of light reflected from its surface. Most gemstone have a glass-like (vitreous) luster. The amount that light rays are bent by a gemstone is its refractive index (RI) which is measured using a refractometer. A gemstone's birefringence is the difference between the minimum and maximum RI.

Gems are generally used to create jewelry and enhance the beauty of objects. They are also collected to be privately admired.

Gems must be relatively hard and fairly resistant to chemical alteration to be able to endure wear and tear. Hardness and durability is one of the key characteristics used to define a gem. The scale of 1 to 10 used to determine gem hardness was created by German mineralogist Friedrich Mohs. Diamond (10) is the hardest and talc (1) the softest. Generally, any gemstone with a hardness less than quartz (7) is considered lacking the durability appropriate for jewelry. Steel is about 6.5, glass is about 5.5, and a finger or toe nail is about 2.5.

Toughness and hardness are different characteristics of a gem. Diamonds is the hardest natural occurring material there is but is not very tough. It's weak atomic bonding along cleavage planes means that, despite its hardness, it can break if it collides with a hard material. Jadeite has hardness of only 7 but is comprised of interlocking fibers, making it the toughest gem.

Fibers or fibrous cavities within a gemstone such as a sapphire reflect the light which creates a star within the stone. A six-ray star has three sets of parallel fibers. Sometimes cutters can create a 12-ray star sapphire.

A break in a natural or imitation gem along a surface that is not a cleavage plane is called a fracture. There are three types of fractures: uneven, hackly, and conchoidal. The conchoidal (shell like) fractures of colored glass is one way to determine a gemstone is an imitation.

The value of a gem is determined by type, color, clarity, size, rarity, popularity and weight. Loose Gemstones are measured in carats (one fifth of one gram).

Most gemstone have a crystalline structure. Crystals have planes of symmetry and are divided into seven symmetry systems. A plane of symmetry causes a crystal to have a mirror image when an imaginary axis is drawn through it. The number of axes, their length, their angle to each other determine the system to which a crystal belongs.

Sapphire is classified as trigonal because it has three planes of symmetry and four axes. Three axes are at 60 degrees to each other in the same plane. The fourth axis is perpendicular and unequal in length to the other three. Spinel is classified as isometric because it has three axes, equal in length, and at right angles to each other.

		The flat surfaces of well formed crystals are called faces. The internal atomic structure of a crystal determines the faces and angles between them. A massive crystal is one comprised of such small, poorly formed crystals that the faces cannot be distinguished. A crystal's habit is the defined sets
of parallel faces and their angles which create its usual shape.

A gem's color is produced when light passes through it. Some colors of the spectrum are absorbed within the crystal structure. Those that emerge combine to give the gem its color. If all the light emerges it will be colorless. This absorption spectrum is used to distinguish between similarly colored but different gemstone and is measured using a spectroscope. A spectroscope could be needed to tell the difference between a ruby colored by chromium and a very similar looking almandine colored by iron.

A gem's ability to split white light into the colors of the spectrum is called its dispersion. A gemstone's dispersion is the difference between its RI for a red ray, the least refracted color, and its RI for a blue ray, the most refracted.

Sometimes it is necessary to use color filters, ultraviolet light, x-rays, or measurements of magnetism to accurately identify gemstone because the color of different gemstone can be very similar to the naked eye and color can be changed by oiling, staining, irradiation, or heat treatment.

Light temperature is inconsistent. An actual gem looks different depending on time of day, geographical location, weather and lighting conditions. A loose gemstone you see on a clear day at 10 am in Bangkok will look different at the same time on a similar day in New York. Information and photos on this site were kindly provided by AJSGems.com Please visit AJSGems.com for fine gemstones.