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The chemistry and physics of gemstones
A stone consists of one mineral or more minerals glued together. A mineral is a solid piece with a uniform chemical composition. Minerals can be divided in the cathegories crystaline minerals, glasses, fibrous minerals and flake minerals.
* In a crystaline mineral, molecules, atomes or ions are arranged into bigger lumps in a regular fasion in three dimmentions, giving also the lump a regular form.
* In a glass, molecules are fixed together in a firm fasion, but the fasion is not regular. The molecules are intermingled in an irregular way. A glass is often formed from a melted mass when that mass is cooled rapidly. It can also form from solutions of a material in water. The stone gets a glasseous structure because the molecules or atoms do not get time to order themselves in a regular way during the solidification process.
If there is no forces from the outside, a glasseous material tend to get round as a ball. If the cooling does not occur uniformly, forces within the material caused by an uneven temperature tend to make the material buckled or wrincled.
Opals are glass-like stones made by deposition of silicon dioxide from aquatic solutions. Obsidian is glasses formed by rapid cooling of molten silicon dioxide from material erupted by volcanos. Amber is recin from old trees, often prehistoric ones, that has been hardend due to chemical processes and transformed to a glass structure. * In fibrous minerals, the molecules are long as threads, giving pieces that can be split up in thin threads.
* In flake minerals, atoms are bounded together in flaky struktures, giving pieces that can be split up in thin lieves.
Fysically you have the following groups of minerals:
* Crystals consisting of atoms bounded together with covalent bridges, so that the whole crystal is actually an enormous molecule. The best examples of this configuration is diamonds .
* Crystals consisting of atoms bounded togeather with polar covalent bridges or ionic bridges, so that the whole crystal is actually an enormous molecule. Most minerals actually are of this kind.
*Crystals consisting of molecules ordered in a regular fasion. The internal binding in each molecule is covalent or polar covalent.
* Glasses consisting of molecules lumped togeather. Opale and Obsidian is examples of this type.
* Fiber minerals consisting of long molecules or very thin crystals lumped togeather. The fasion maybe regular so that the fibers are aligned in the same directions or irregular. If the fasion is regualar, the piece can easily be split into thinner fibres.
* Flake minerals consiting of flake-formed molecules stacked on the top of each other. Flake minerals can easily be split into steadily thinner lieves.
Water or sometimes other foreign molecules may be trapped into a crystal or glasseous framework and become a regular part of the structure - crystal water.
Foreign molecules that do not form a regular part of the framework may also be dispersed in the crystal or glass.
The stable color of a mineral is either due to the colour of the basic crystal framework, or due to foreign molechules.
Colour can also be due to breaking of the light in a crystal, like the colour play in diamonds, or be due to interference in the framework. Colour due to these effects will generally vary with the light falling onto the stone or with the angle one sees the stone from.
Chemically minerals can be divided in these main types:
* Pure elements, like diamonds that consist only of carbone.
* Compositions of one or more metal elements (or half-metal elements) and an electro-negative element like oxygen or halogen. This is the most abundant type.
* Compositions of elements of more or less the same elctronegativity or electropositivity.
All these properties of minerals also are properties of gemstones. What distinguishes a gemstone from other minerals are actully these factors: They look nice, they are hard and durable and they have a consistance that makes it possible to cut and form them.