Since it is October and OPAL is it's birthstone, I'm going to post everything I can about this semi-precious stone. Opal is the national gemstone of Australia. It is an amorphous form of silica, a mineraloid. 3% to 21% of the total weight of opal is water.
The name opal has many origins. It was thought to be derived from the Sanskrit word "upala", meaning "valuable stone". According to wikipedia, the word opal comes from the Greek word, opillos meaning "to see a change in color".
Opal's internal structure makes it diffract light; depending on the conditions in which it formed it can take on many colors. The light bounces around inside the tiny microscopic silica spheres within the stone, causing the diffraction of light and the 'prism' or rainbow effect. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. If opals are lacking the typical play of colours, they are simply named "Common Opal".
Thanks to: Opalsdownunder.com, Wikipedia and Gemstone.org for the information