Opals are beautiful gems that come from seasonal rain that drench grounds that are very dry, including Australia, and other desert locations. 

The showers create a soaking that goes all the way down to; the rock that’s underground. The silica that’s dissolved then goes downwards. 

During these dry periods, a lot of the water tends to evaporate, which leaves some solid silica deposits within the cracks that are there in between sedimentary rock underground.  The deposition of silica are what creates opal. 

Opal is known in many cases for the rainbow-like colors that are there. This is known as a play of color.  There are two types of opal, and those are common and precious. Precious types of opal show the play of color, while the common ones don’t. 

Play of color happens in the precious ones due to the sub-microscopic spheres and the grids that are stacked in a pattern, such as ping pong ball layers that are within a box. When the light waves travel between each of these the waves then diffract.  While they bend, they then break these off into different spectral colors, which are colors seen in the rainbow.  The play of color is the ultimate result of this one. 

The colors as seen in these do vary with the sphere sizes. Some spheres that are about a tenth of a micron, which is one ten millionth of a mere meter in diameter, create some violet. 

The ones that are about two tenths of a micron create red.  Sizes that are in between these are what’s responsible for the vivid rainbow colors. 

Opal comes in the following five categories: 

• White opal, which is translucent or semitranslucent, with some play of color that happens against a gray or white background.
• Black that is done against a black sort of background, and usually ranges from translucent to opaque
• Fire, which is brown, yellow, orange, or red. 
• Bondar: a play of color that’s usually fragments of different rocks that are a part of this 
• Water or crystal: a play of color that has a very clear background