Gem

A gem is a Semiprecious stone, invaluable or decorative or any matter very hard or coloured having the aspect of these precious stones and used like ornament.

To deserve the name of gem , this matter (Mineral, Rock or an organic substance such as Pearl or Amber) must be beautiful, especially by its color. It must be not very liable to deterioration, and enough solid to survive a constant use or handling, without striping themselves or damaging themselves.

Raw stone torn off with the ground by the minor, a gem is often cut by the Lapidaire to finish gone up on a rings, in Boucle of ear or all other ornaments of Joaillerie. It can be natural, treated or artificially manufactured (synthetic stone).

The gemmology is the science of the gems.

Invaluable stones

At present only the four following gems are classified among the invaluable stones:
  • the Diamond, transparency, (pale blue, white…),
  • the emerald, transparent, (green dark),
  • the Ruby, transparency, (clear red with scarlet),
  • the sapphire, transparency, (light blue with means),

Fine stones

In the past called semi-precious, they gather the transparent gems not classified among the invaluable stones:
  • the Aquamarine, transparent, (blue pale green)
  • the Amethyst, transparent, (crimson)
  • the Citrine ,
  • the Rock crystal, transparency, (clearly)
  • the Cordierite,
  • the géode,
  • the Garnet-red , translucent, (red, maroon, green or purple),
  • the Peridot,
  • the Tanzanite,
  • the topaz, transparent, light blue with bed,
  • the Tourmaline, transparent, (green, blue, chestnut or pale red),
  • the Zircon, transparency, (colorless, blue, green pale with green, brown, etc).

Decorative stones

Also called stones of imagination , they are opaque or translucent:
  • the Agate, translucent, (striped maroon, blue, white, red),
  • the Alexandrite, variety of chrysoberyl.
  • the Amber, transparency, (various nuances of yellow or gold),
  • the azurite, opaque, (mottled dark blue),
  • the Chalcedony, opaque, (white),
  • the Cameo,
  • the red, opaque Coral, (red scarlet),
  • the Cornaline, red with brown-red
  • the Hematite, opaque, (gray black),
  • the Jade, translucent, (green pale, dark, green and white),
  • the Jet, opaque, (black),
  • the Jasper, opaque, (blue, with black reflection with chestnut),
  • the Lapis lazuli, opaque, (light blue and dark, yellow spots),
  • the Malachite, opaque, (striated, green clearly and dark),
  • the Marcassite,
  • the Obsidian, opaque, (black),
  • the onyx, transparency, (pure black, pure white, or bands),
  • the Opal, translucent, (blue pale with spots green and gold),
  • the Pearl, translucent, (white, yellow, pink, black),
  • the Pierre of the moon, translucent, (white with pale blue gleam),
  • the Sard, opaque, (orange with red chestnut),
  • the Spinel, transparency, (red, chestnut, dark green),
  • the turquoise, opaque, (blue pale green).

Synthetic stones

These stones are produced industrially, generally in order to imitate required natural stones, like the Moissanite.

A specific regulation exists in certain countries, obliging the salesmen to clearly specify the synthetic nature of the stone. Thus in France, the decree 2002-65 of January 14th, 2002 relating to the trade of the gemmeous stones and the pearls specifies that: Article 4: The following qualifiers respectively supplement the denomination of the matters and products mentioned below:
(...)
- " synthétique" for the stones which are crystallized or recristallized products whose manufacture caused completely or partially by the man was obtained by various processes, whatever they are, and whose physical properties, chemical and of which the crystalline structure corresponds essentially to those of the natural stones that they copy;
(...)
Use of the terms: " élevé" , " cultivé" , " of culture" , " vrai" , " précieux" , " fin" , " véritable" , " naturel" is interdict to indicate the products enumerated with the present Article. (...)

Inclusions

Few gems are perfectly pure. The majority, indeed, contain foreign bodies or present various accidents of crystallization. These accidents are regarded as inclusions, that it is necessary to take care not to describe like “defects”, because their presence inevitably does not involve a depreciation of the gem.

Inclusions follow strict laws and can give information as for the formation and with the types of layer of the invaluable, fine or decorative stones.

They are characteristics of identification: each gem concealing its own inclusions, it is possible to gather the whole of the photographs of the inclusions found in a particular species (for example topazes) and to order them.

Inclusions are relatively frequent in minerals; they are is same species (inclusion of diamond in diamond, for example) or foreign (gold inclusion in quartz, for example). If small they are, inclusions can bring invaluable indications on the formation of the surrounding crystal, called “crystal host”.

The minerals included can be older than him, and be simply included at the time of the growth. They can as be formed at the same time as the crystal host, which, following a faster growth, included them in its mass. There exists, moreover, of inclusions which are more recent than the crystal host: they come from liquids which were introduced by cracks into the crystal.

Here some quartz with inclusions:

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