-
Like opals, the "silica gel" in these little packets are made of silica.
-
Both "silica gel" and opals can hold a large amount of water.
-
Gem quality opals usually contain less than 10% water.
|
Formation of Opals
Opals are chemically similar to quartz, amethyst, and glass. However, unlike most gemstones, opals are not composed of crystals. They are primarily composed of silica and water. Opals are created when weathered silica seeps into fissures as gel. At this point, the silica gel contains a large amount of water and would flow just like a glacier. You can see what opals would look like during this early period of formation by mixing sodium silicate, more commonly known as “Water Glass” in pharmacies, with vinegar. As the water evaporates, the gel hardens. How quickly the water evaporates determines the stability of the opal. Just like dried mud, cracks will form in the opal if the water evaporates too quickly. These unstable opals are common worldwide and are not considered to be gem quality. If the water evaporates slowly, the opal will be stable, and if it has nice fire, it will be considered gem quality. Gem quality opals usually contain less than 10% water. If an opal has a small concentration of water, an evaporation event capable of causing instability (fractures) is less likely to occur.
|
The Cause of Color in Opals
The cause of the colorful patches of fire in opals is often misunderstood. A common myth is that the fire is caused by the water contained within the opal. The misconception is that the colors are caused by diffraction just like a rainbow. This is not true. Opals are composed of tiny silica spheres. When the spheres are the same size and are arranged in a three-dimensional grid, negatively charged areas are created in the gaps between the spheres. If those negatively charged areas are as big as a wavelength of visible light, then colors will be visible. One reason opals with red fire are so popular is because an opal with red fire will also display every other color. Red light has the largest wavelength followed by orange, yellow, green, blue, and violet. If an opal displays red fire, then that means the negative areas between the spheres are as big as the largest wavelength of visible light. As the opal rotates slightly, the colors change as the negative areas become gradually smaller from the viewer’s perspective.
|
 |
