Dendritic opal (moss opal, merlinite) - white, opaque and is permeated by a network of fine black dendrites (oxides or hydroxides of iron or manganese). Opal is a mineral, a hydrogel with fluctuating water content, whose chemical formula is SiO2. n H2O. Water is present most often between one and three percent by weight, but in some cases it can reach up to 20%.