Purple and white "Tiffany" stone, also known as opalized fluorite. 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 white plastic support.
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 1/8 inches (2.9 cm) wide, 1/2 inch (1.3 cm) deep and 7/8 inch (2.2 cm) tall.