Semi Precious Gems
The oldest Book created by mankind ‘Rig Veda’ describes number of Gemstones and gives references which indicate that Gemology or Ratan Shastra was a well developed science and that the Royal family and the Rich people kept their wealth in Gems and there was a strong desire in the general public to acquire Gemstones.
Gemology has three parts:
Identification of Gemstones,
Cutting and polishing of Gemstones (Lapidary Works), and
Upkeep of Gemstones.
Identification of Gemstones
Gem identification is a very complex science as the machines are not able to give accurate results. The personal observation of inclusions by the Gemologist is the only think that leads to a conclusive result. The physical constants as measured by various machines can be miss-leading as the Lab. Made gemstones have same physical constants as their NARURAL counterparts.
In this science the role of experience is far greater than the role of theoretical knowledge of Gemology.
Cutting and polishing of Gemstones
Cutting and Polishing of Gemstones is a very intricate process as extracting the maximum value out of Rough Gemstone and then making a piece of Gem come out with best possible colour, shine and size is the aim of this science.
This is a again an area of experience – experienced artisans have an intuitive understanding of Gemstones and are able to bring out the best value out of a piece of Rough Gemstone.
The way various Gemstones are Cut and polished is different depending on their hardness and rate per carat.
Upkeep of Gemstones
Upkeep of Gemstones should be very carefully done. If Gemstones of different hardness are kept together they may scratch each other and the Gemstones may be rendered valueless. The there are various chemicals and detergents that can harm a Gemstone. There are number of factors that have to be kept in mind while storing and setting Gemstones.
Indian Gemology or Ratan Shastra divides Gemstones in three categories namely:
Precious Gemstones are nine in number. They are Diamond, Emerald, Ruby, Blue Sapphire, Yellow Sapphire, Coral, Pearl, Cats Eye and Hessonite.
Semi – Precious Gemstones
Semi Precious Gemstones are thirty four in number namely:
Amethyst, Aquamarine, Black Onyx, Black Rainbow, Black Star, Blue Topaz, Garnet, Golden Topaz, Green Amethyst, Green Fluorite, Green Garnet, Green Onyx, Green Tourmaline, Iolite, Jade, Kyanite, Labroloite, Lapis Lazuli, Lemon Topaz, Malachite, Moonstone, Opal, Pink Tourmaline, Red Onyx, Rhodolite Garnet, Rose Quartz, Smoky Topaz, Star Ruby, Sang e Maryam, Tiger’s Eye, Turquoise, White Coral, White Quartz, White Rainbow, White Topaz, Zircon.
There are forty-one Gemstones in this category
Monochromatic light: This is a light of one wavelength only. The standard monochromatic light used for optical measurements by Gemologists is the yellow light transmitted by a Glowing Sodium Vapour. This is actually created by two identical wavelengths.
Nanometre: (nm) This is a unit of length previously known as a millimicron. It is equivalent to 10-9m or to 10 A.
Nicol prism : This is a special prism for producing polarized light rays. Light entering the prism is split into two polarized rays-the ‘ordinary’ ray is totally reflected at the balsam layer while the ‘extraordinary’ ray is able to pass through the prism. In a Petro logical microscope two Nicol prisms are incorporated.
Opalescence: This is a term used both for the “milkiness” of common OPAL as well as for the milky iridescence seen in the PRECIOUS OPAL.
Ordinary Ray: In uni-axial stones, the ray of light which travels with constant velocity in any direction within the crystal is known as Ordinary Ray.
Organic: That which is Produced by living organisms. In Gemology this refers to Pearls and Corals.
Orient: The special sheen and iridescence displayed by Natural Pearl. This can not be copied or created artificially.
Phosphorescene: Phosphorescene is an effect differing from fluorescence in that the luminous glow persists for a long period even after the removal of the exciting radiation device.
Pleochroism: This term relates to embracing Dichroism and the effects shown by some bi-axial Gemstones and minerals in which three distinct colors can be seen by means of Dichroscope.
Polarized light: Light vibrating in one plane only. Polarized light can be produced by means or reflection from glass plates at certain angles, by absorption of one of the two polarized rays passing through dark-colored tourmaline plates or Polaroid sheets, or by means of a Nicol prism.
Polaroid: The sheets of plastic material containing ultra-microscopic crystals of quinine iodo-sulphate or other material which have the property of transmitting only one polarized ray, the other being almost entirely absorbed by the crystals. ‘Polaroid’ sheets provide a light and inexpensive substitute for Nicol prisms in the production of polarized light.
Radioactivity: The spontaneous emission of particles or rays accompanying the gradual breakdown of certain unstable atoms, mostly of high atomic weight. The best-known examples of radioactive elements are radium, thorium, and uranium. The emissions may be of three kinds: alpha particles, which are helium Nuclear; beta particles, which are electrons; and gamma rays, which are akin to X-rays of very short wavelength. The first two have little penetrative power, but gamma rays are more penetrating than the hardest X-rays.
Reconstructed stones: Stones made during the nineteenth century from chips or natural ruby by fusing them together under a blowpipe flame. These so-called reconstructed rubies have been superseded for over fifty years by synthetic stones made by the Verneuil process, for which the term should not be used. Pressed amber is the only truly ‘reconstructed’ gem material at present being made.
Reflection: A ray of light incident on a polished plane surface is reflected by it in such a manner that the angle of reflection is equal to the angle of incidence. The proportion of reflected to refracted light in transparent substances increases with the refractive index of the substance and with the angle of incidence. For example, 17 per cent of the light falling at perpendicular incidence on the surface of diamond is reflected, whereas with quartz less than 5 per cent is reflected under these conditions.
Refraction: When a ray of light passes from air into a denser medium such as a gemstone its velocity is lessened and as a consequence the ray no longer follows its original path but is bent or refracted to follow a direction more nearly perpendicular to the surface between the two media.
Refractive index: A quantity representing the refracting power of a medium, in which air = 1.00 is in practice taken as a standard. The refractive index of a medium may be defined as the ratio of the sine of the velocity of light in air to the velocity of light in the medium, or as the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light passes from air into the medium.
Refractometer: An instrument designed for measuring the refractive indices of various substances. In Gem Testing it is used
Sheen: Sheen is the appearance caused by the Reflection of light from structures inside a stone, e.g. Moonstone.
Silk: The fine intersecting rod-like crystals or cavities typically seen in Burma ruby, which give a silky sheen by reflected light.
Specific gravity: The weight of a substance compared with the weight of an equal volume of pure water at 4.C
Spectroscope: An instrument which resolves light into its component wavelengths by refraction through prisms or diffraction by grating.
Spectrum: A band of light showing in orderly succession the rainbow colures or isolated bands or colors corresponding to different wavelength, as seen through a spectroscope, or photographed in a spectrograph. The visible spectrum is only a small region in the vast spectrum of electromagnetic waves, which extend from the longest radio waves to the minutely short waves (gamma rays) emitted by radio-active elements. An emission spectrum is produced by the glowing vapor of elements (particularly metals) and consists of bright narrow lines of definite wavelength. A continuous spectrum is the band of all the Rainbow Colors, Red, Orange, Yellow, Green, Blue and Violet merging in Gemstones one into the other, produced by all incandescent Gemstones. An absorption spectrum is the series of dark bands crossing a continuous spectrum, seen when white light has been transmitted through a colored vapor, liquid or a Gemstone or mineral.
Step – or Trap-cut: A popular style of cutting used for colored Gemstones and sometimes for Diamond also, in which a series of facets both above the below girdle have edges parallel to those of the rectangular table facet.
Symmetry: There are three ‘elements of symmetry’ recognized in crystallography:
1. Planes of symmetry,
2. Axes of symmetry, and
3. Centre of symmetry.
On this basis Gemstone Crystals are divided into 32 classes. These Gemstones in turn are grouped into seven systems.
Synthetic Gemstones: Manufactured stones which have essentially the same chemical composition, crystals structure, and properties as the Natural Mineral they represent. The only way to differentiate between a Natural Gemstone and a Synthetic Gemstone is to study its inclusions under magnification.
Transition elements: This term applies to a series of metals occupying contiguous positions in the periodic table of the elements, which, due to their atomic structure, absorb light and impart characteristic colors to minerals in which they occur as a major or even an extremely minor percentage. In order of increasing atomic number, these are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper.
Transparency or diaphaneity: The degree to which light is transmitted through a substance. A Gemstone is termed transparent when objects can be clearly seen through it, as through glass; e.g. Quartz, Diamond, Sapphires, Topaz etc. Where little light is transmitted but no clear outlines can be discerned, the Gemstone is termed translucent; e.g. Jade, some varieties of Agates etc. Where no light can pass through, the Gemstone is opaque; e.g. Malachite, Lepis Lazuli etc.
Twin crystals: Two or more Gemstones or a mineral which have grown together in a symmetrical fashion in such a manner that the parts of the twin have some crystallographic direction or plane in common, but others in reversed position are known as Twin Crystals.
Ultra-violet light: Invisible rays of wavelength somewhat shorter than those of visible violet light. Conveniently classified as long-wave or ultra-violet light, i.e., from the 365nm line of mercury, and short-wave ultra-violet, i.e., the 253.7nm mercury line.
Uniaxial: Minerals and Gemstones having a unique direction of single refraction – i.e. one Optic Axis. Tetragonal, Hexagonal, and Trigonal crystals are uni-axial.
X-rays: Electro-magnetic radiations having the same nature as visible light but of much shorter wavelength. X-Rays are capable of piercing though soft materials.
Extensively used in Gem Testing as an additional tool while testing Organic Gemstones like Red Coral & White Coral and Pearl.