Opal is a soft amorphous silicate gel. Some opal displays a property called opalesence. Opalescence is a kind of iridesence which occurs when water gets caught in small microstructures within the material. Not all opal is opalescent, but it is still opal.
This opal has dendrites (structures that look like plant roots or the branches of trees) within it. The dendrites are formed of a manganese oxide mineral called psilomane. Dendritic opal is sometimes also called Merlinite.
Weight, 107g. This photograph is of the item offered for sale.