Opal is one of the most beautiful gemstones. Its structure consists of the smallest spheres of silica (globules) and water between them. These gems are distinguished from the rest by a remarkable unique optical effect – opalescence. The transmitted light is divided into colored rays, as a result of which we observe an interesting play-of-color. Optical properties allow to observe different pure colors in the form of bright spots that can change their color depending on the angle of view and lighting in one sample. The color of opalescence depends on the size of the silica globules. With a size of about 0.1 microns, violet colors are obtained, with a size of 0.2 microns – red. The sizes between these values of the globules give all the remaining colors of the rainbow.