This is a polished piece of labradorite
that has been meticulously polished into the shape of a skull. The brilliant blue/green/gold/purple colors have nice play in the light. The labradorescence is spotty, however there are two spots of excellent purple color play. The underside of the skull and the nostrils have been hollowed-out, making the specimen closer in resemblance to a real skull.
Labradorite is a feldspar mineral that is most often found in mafic igneous rocks. Some specimens of labradorite exhibit what is called a "schiller effect", which is a strong play of iridescent blue, green, red, orange, and yellow colors. Labradorite is so well known for these spectacular displays of color that the phenomenon is known as "labradorescence." Specimens with high quality labradorescence are often polished and used as gemstones.
The labradorescence is not caused by the colors being reflected on the surface of the specimen. Instead, light enters the stone, hits a twinning surface within the stone, and reflects from it. The color seen is the color of light reflected from that twinning surface. Different twinning surfaces within the stone reflect different colors of light. Light reflecting from different twinning surfaces in various parts of the stone can give the stone a multi-colored appearance.
Labradorite is named after where it was first found in Labrador, Canada. Today, the most prolific deposits and the source of most commercially available labradorite occur in Madagascar and Russia.