Opal is an amorphous substance, hydrated silicon dioxide (SiO2.nH2O), with a variable amount of water (3-21%). There are several opal varieties nonetheless the precious opal represents one of the most valuable ones thanks to opalescence, a magic light reflection of sparkling colourful pastel colours. Colour changing is caused by a refraction and dispersion of light on fine layers with different content of water, minute admixtures of other minerals or in micro-fissures. The precious opal is the most famous gemstone of Slovakia. It can be found in fillings of fissures in Late Tertiary volcanic rocks – andesites in areas between Červenica and...