Cabochon: A gem that is cut round without facets into the shape of a smooth polished dome.
Semi-precious: Traditionally, the precious gemstones included diamond, ruby, sapphire and emerald. Semi-precious gemstones included everything else. Other gems have also been labeled precious at times, including opal, amethyst and pearl. No longer are gems referred to as precious and semi-precious by many in the gem trade.
Most gemstones are naturally made. But others are man-made such as gold stone. Gems can also be treated in many ways.
Treated Gemstone: A stone can be heated, dyed, irradiated, or stained in order to improve the color or the clarity. A stone is also considered treated if it has had cracks or fractures concealed with filling material.
Fracture Filling: Small cracks can interrupt the flow of light through the stone, creating white or "dead" spots in the color. Fractures are sometimes filled with oil, wax, glass, epoxy, or borax. These materials allow the light to pass through smoothly.
I often speak of inclusions in my descriptions of cabochons.
Inclusions: Inclusions are characteristics which are inside a stone or extend to the inside from the surface. They are normal within gemstones and most do not affect the structural integrity of the stone. These include:
- Clouds- hazy areas in a stone
- Needles- long thin inclusions, also called growth tubes
- Cracks- also called fractures, breaks or feathers. Straight flat cracks are called cleavages.
- Halos- circular fractures
- Growth or Color Zoning- uneven color distribution in a stone
- Cavities- holes or indentations extending from the surface into the stone
- Chips- notches or chips
Calibrated: A calibrated gemstone is one that is cut to a standard size that is uniform with commercial jewelry settings. The measurement is in millimeters, such as 10mm round, 16x12mm oval, etc.
That is all for today's lesson on common gemstone and cabochon terminology. I hope this is helpful for the novice jeweler and anyone just interested in gems.