This is a beautiful thumbnail size Demantoid Garnet from Tubussis, Namibia 

Dimensions: 3cm (1.1”) x 2.5cm (0.9”)

Perky Display Dimensions: 3.5cm (1.3) x 3.5cm (1.3”)

Weight: 13.5 grams

Demantoid is the green gemstone variety of the mineral andradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet

the chemical formula for Demantoid Garnet is:

Ca₃Fe₂(SiO₄)₃

With chromium substitution as the cause of the demantoid green color. Ferric iron is the cause of the yellow in the stone.