The name opal is derived from the Latin word opalus, meaning seeing jewel. According to another theory the word opal comes from upala, a Sanskrit word simply meaning a stone. 

 

Amorthous silica, SiO₂ + nH₂O, A solidified jelly so it has no crystal structure and contains up to 13% water. Precious opals are translucent to transparent and are distinguished by a combination of milky to pearly opalescence and an attractive play of many colors. These colors flash and change as a stone is viewed from different directions are caused by interference of light along minute cracks and other internal inhomogeneities. 
Opal is extensively used as a gemstone. Various forms of common opal are widely mined for use as abrasives, insulation media, fillers, and ceramic ingredients. 

 

The most striking quality of opal is its ability to refract and reflect specific wavelengths of light. The interplay of different wavelengths of light gives opal its unique visual appeal, and makes it one of the most sought-after gemstones in the world at all times. 
Precious opal has been a gemstone since Roman times. Opal was included among the noble gems and was ranked second only to emerald by the Romans. A beautiful opal called the orphanus was set in the crown of the Holy Roman Emperor. It was described "as though pure white snow flashed and sparked with the color of bright ruddy wine, and was overcome by this radiance." Opals are also set in the crown jewels of France. 
Most opal is more than 60 million years old and generally dates back to the Cretaceous period when dinosaurs roamed the earth. Most of the precious opal marketed in ancient times was obtained from occurrences in what is now Slovakia.