This clear form of opal is often called "jelly" opal. This specimen has windows of opal in a white matrix but has the added touch of some small touches of play of colour in the white area. Hard to capture in photos but I've tried. Most people think of flashy precious opal from Australia, but opal actually occurs in various colours and around the world. The non-flashy type is often referred to as common opal. This specimen is discreetly glued to a polished black onyx base.
The final photo shows examples of all the different colours and forms of opal that I have on my web site (at least until some sell). Just put "opal" into the search box to see them all.
1/2 inch (1.3 cm) wide, 3/8 inch (0.95 cm) deep and 5/16 inch (0.8 cm) tall.