Colors are the result of the absorption of certain wavelengths of visible light.
When a specific wavelength of light passes through a gem, or is reflected from its
surface, what we see are the wavelengths that are not absorbed. So, when we see
a red gemstone, in actual fact that gemstone is every color but red. The absorption
of certain light wavelengths in some
gemstones is caused by the inclusion of impurities; these impurities add
color to minerals.
The color in Amethyst was originally thought to be uniquely due to
the presence of the mineral impurity of iron. But the impurity theory did not add
up when Amethyst was clearly observed to have an unstable coloration, which faded
after exposure to heat and light sources. The fading of color in this way is contradictory to the impurity theory, as other gem types such as Ruby are routinely heated up to as much as 2000 degrees Celsius, which instead of weakening the color intensifies it. Moreover, heated at high temperatures Amethyst was observed to loose it's purple nuance altogether, shifting to a yellow color. In fact, this is the source of much of the world's Citrine.
On closer reexamination Amethyst was observed to gain its coloration, not only from the impurity of iron, but also from natural atomic phenomenon present within the gemstone called 'Color Centers.' 'Color Centers,' are molecular sized imperfections in crystals that cause the absorption of certain light wavelengths, giving a gem its subsequent color. These 'Color Centers' can be created or modified by processes of irradiation and heat. The initial coloration in Amethyst was created by radiation when the mineral was
formed deep within metamorphic rocks under the Earth's surface.
gemstones, at some point in their billion year lives, have been exposed to radiation in one form or another. In the case of
Amethyst , the forming of 'Color Centers' by natural radiation took place when high-speed electrons pierced the mineral's atomic crystal structure, hitting
smaller molecules and displacing them. When these molecules were displaced from their regular positions, their place was taken by one of the electrons, which became trapped in the vacant space. This trapped electron absorbed light energy, which caused it to get excited and jiggle. The light energy was then dissipated through
the motion of the excited electron, and with it a subset of color wavelengths from the light energy was used up. What we see as the color of an
Amethyst is what light energy wavelengths remain that have not been used up by the excited electron. This is the creation of a 'Color Center,' the phenomenon
that gives Amethyst, topaz and some colored diamonds their color.
For the most part Amethyst is sold much the same as it appeared from the ground but sometimes.
- . Very dark natural amethysts are heated to produce lighter shades. This makes them more desirable and also allows for consistent colors that can be matched for use in jewelry.
- . Very light purple amethyst can be heated to produce citrine and, to a lesser extent, green quartz.
- . If the amethyst
contains color zoning, (a color concentration that conforms to crystallographic growth patterns) it is heated to give these areas a dramatic color change effect.