This is a beautiful thumbnail size Demantoid Garnet (Andradite) from Antetetezambato, Ambanja District, Antsiranana Province, Madagascar 

Dimensions: 2cm (0.7”) x 2cm (0.7”) 

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

Weight: 7.1 grams

Evidence of Rainbow inside crystal (See photos for reference) 

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.