A gem is a natural material which can have a mineral, animal, vegetable or meteoric origin. The characteristics that give value to the gems are three: beauty, hardness, and rarity. The beauty of the gems initially could be considered a subjective concept, but it is defined by the following objective components: color, gloss, transparency, and special optical effects.
To speak of color in gems, we must refer to their abilities to pass rays of light by reflection or transparency. The human eye captures the waves that make up the ray of light that have been reflected by the gem and the brain makes an interpretation of them, according to its wavelength, and classifying in colors.
The eye can only capture the waves of the visible part of the electromagnetic scale. One-color gems are those that owe their color to elements of their chemical composition; therefore gems belonging to this group always have the same color, varying only the shades.
Moreover, gemstones lacking color in their purest form, are called allochromatic, meaning trace amounts of impurities can tint crystals as they can take different colors depending on the chromophore that is integrated into their composition.
The color is a fact that helps, alongside the physical and optical properties, to identify both mineral species and their varieties. The richness in color of a gemstone is determined by the intensity, stones of bright colors being the most desirable.
The Ministry of Mines and Energy of the Republic of Colombia (2003) defines it as “a related characteristic of reflection of light that strikes the surface of a mineral” (page 21).
- Waxy: related to the brightness of the wax, typical of minerals that reflect a reduced amount of light.
- Pearly defines the brightness possessing the pearls.
- Resinous: typical of organic gems such as Amber or Copal.
- Vitreous: related to the brightness of the crystal, it is typical of minerals such as Quartz.
- Adamantine: typical of the diamond.
- Metallic: reflective brightness in metals, in gemological materials is found in the Pyrite.
Unlike the brightness, this effect occurs within the stone due to the presence among others of channels, materials, fibers, light interference. The light penetrates the stone, finding these barriers or peculiarities. When the light is sent back to the outside, it shows the optical effects that are named either by the stone that tends to come or the object that recalls the appearance thereof.
- “Cat’s Eye” effect, or Chatoyancy: the light wave produced by this effect, reminiscent of the eyeball of a cat, is caused either by the presence of inclusions or channels oriented in one direction. For the effect to become apparent, the gem should be carved in cabochon (style carving with the upper in the shape of a dome). It is apparent in the quartz, beryl, tourmaline, and the apatite, among others.
- Star Effect or Asterism: In this effect, we can notice a star of four to six points, brightly lit, which is displaced when moving a gem in a cabochon shape. Some of the gems in which we can observe this effect are the corundum, pink quartz, and the diopside.
- Adularescence: this effect is a bluish or whitish glow in the gem. It can be found in the orthoclase, adularia, and moonstone and some albites.
- Iridescence: is the appearance of certain colors of the rainbow, on cracks or cleavage, due to interference or diffraction of light. We can observe it in the rainbow quartz and topaz.
- Labradorescence: consists in the iridescence of the gem in a bright metallic color, appears especially in blue and green gems.
- Opalescence: This effect is caused by light reflection on short wavelengths, which produces a bluish, milky or pearly sparkle in the gem. It shows in the opal and the milk quartz.
- Pearl Luster: Presented by reflection of light in the surface layers of the aragonite in pearls.
Its importance lies in the way it relates to the duration in time of the gem.
It is understood as an ability of gems to resist dents or scratches with other materials. Its importance lies in the way it relates to the duration in time of the gem.
That is why, in the “Mohs Scale” a scale of mineral hardness, starting with Talc given the number 1 and ending with the number 10 the Diamond. Each mineral scratches the one which has a lower number and can be scratched by a number equal to or higher than its own. The following table shows the Mohs scale.
Hardness Mineral Remarks
|1||Talc||It can be easily scratched with a fingernail|
|2||Gypsum||It can be scratched with more difficulty with a fingernail|
|3||Calcite||It can be scratched with a copper coin|
|4||Fluorite||It can be scratched with a knife|
|5||Apatite||It can be hardly scratched with a knife|
|6||Orthoclase||It can be scratched with steel sandpaper|
|7||Quartz||It scratches the glass|
|8||Topaz||It scratches all the above|
|9||Corundum||Rubies and Sapphires are forms of Corundum|
|10||Diamond||It is the hardest mineral of all|
While other hardness scales have been established based on other principles; given the simplicity of the method, the “Mohs Scale” prevailed and is now the most applied in Mineralogy.
In the world of gems, there are some that are common and others of immense rarity, which in commercial terms translates into greater value.