Naturally yellow common opal from Tanzania. The colour is not quite as vivid as it looks under my photography lights but it is defintely yellow. 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.
3/4 inch (1.9 cm) diameter and 1/2 inch (1.3 cm) tall.