Naturally yellow common opal from Tanzania. 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 custom wood 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 inch (2.5 cm) wide, 11/16 inch (1.75 cm) deep and 3/4 inch (1.9 cm) tall.