| Physical Properties of Gems |
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| Hardness: The power a stone possesses to resist abrasion when a pointed fragment of another substance is drawn across its smooth surface without sufficient pressure to develop cleavage? Harder stones will scratch softer ones. Stones of the same hardness may scratch each other (a diamond can scratch a diamond). The Mohs scale is used for gemstone hardnesses. This scale is purely relative. Mohs Scale 1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase feldspar 7. Quartz 8. Topaz 9. Corundum 10. Diamond Specific Gravity: The specific gravity of a gem is its weight when compared with the same volume of water at a temperature of 4 degrees Celsius. It can be measured with either hydrostatic method or heavy liquid method. The denser the minerals in the gemstone are, the heavier the weight or specific gravity will be. Heavier gemstones are usually harder as well. Refractive Index: When light passes through a substance, it bends.This is expressed as the ratio of velocity of light in the air to its velocity in the substance.This is measured with the help of a refractometer.This is one of the important property for differentiating gemstones. some gemstones have single refraction and some have double refraction. This is determined with the help of an instrument known as Polarimeter and it relates to axis of gemstones.Gemstones can be Uniaxial or Biaxial. Magnification: This is viewing the gemstone through a microscope or for most purposes a 10x lens suffice.This picture gives lot of information about gemstone.By naked eye it is not possible to make any conclusion about a gemstone. Cleavage: is the splitting of gems and minerals along one of the planes related to the stone's structure. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture. Fracture: is the way a stone breaksWhen a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture. Tenacity or Toughness: is the ability of a stone to withstand pressure or impact. Minerals which crumble into small pieces or a powder are said to be brittle. If a gem bends but returns to its original position, it is said to be elastic (mica, nephrite, jadeite); these minerals are tough and difficult to break. Magnetism and Electricity: Those stones which are attracted by a magnet are considered magnetic, such as magnetite and hematite, which contain iron. Most minerals and gems are poor conductors of electricity. Good natural conductors include native metals and minerals with a metallic luster (pyrite). Natural blue diamond is a semi-conductor. Thermal Conductivity: Some stones are good conductors of heat. Quartz, which draws heat away from the body when held and thus feels cold to the touch. A poor thermal conductor, such as amber, feels warm to the touch because it does not conduct heat away from the body. The surface of a genuine gemstone will de-mist more rapidly than that of glass or an artificial stone. Crystal Systems: Although traditionally crystal systems are not part of the physical properties table of gemstones, but most mineral gemstones are crystaline. Crystal systems are classifed into 7 different systems, according to the "minimum symmetry" of their faces. Hexagonal/Trigonal: This systems share the same axis of symmetry Monoclinic: has a minimum symmetry of the one two-fold axis. Cubic or Isometric: Crystals have the highest symmetry. The minumum symmetry is four-fold axes. Tetragonal: This system is defined by one four-fold axis. Orthorhombic: The minimum symmetry of this system is three two-fold axis. Triclinic: Crystals of this system have no axis of symmetry, so gemstones within this system are the least symmetrical. Note: Gems are cut in different styles, based on their harndess, cleavage, refractive index, fracture and crystal type. Click here to see the different types of cuts. |