opaali
Opal, or opaali in some languages, is a hydrated amorphous form of silica (SiO2·nH2O) that ranges in color from milky white to transparent to brilliant hues. Its most distinctive feature is the play of color, produced when light interferes with the microscopic arrangement of silica spheres within the gem. The water content, typically about 3 to 21 percent by weight, influences its appearance and stability.
Formation occurs when silica-rich solutions fill voids in rocks such as volcanic ash beds or sedimentary formations.
Varieties include precious opal, which displays play-of-color across its body, and fire opal, which is transparent
In jewelry, opal is valued for its color and iridescence but is relatively soft (Mohs hardness about