What are star rubies and sapphires?

Rubies and sapphires often have the effect of asterism sometimes in everyday speech called "star". Asterism is a special case of iridescence in the shape of a light star formed due to reflection from regularly located internal inclusions and inhomogeneities. Most often asterism is associated with the inclusion of rutile in rubies and bohmite in sapphires, but the reasons that cause this kind of iridescence may be in other heterogeneities of the mineral composition and stone structural configuration. Nowadays artificial creation of the effect of asterism in corundums as a result of special diffusion is widespread.