Lapis lazuli

Lapis is a semi-precious gemstone, known and used as far back as Pharonic Egypt. It is a compound of various other minerals and, in its finest form is intense radiant blue, occasionally with golden pyrite flecks. It takes excellent polish and has been widely used for jewellery, and in mosaics. Ground, it is a major constituent in the pigment Ultramarine.

Accordingly it is a period stone, and was available in medieval Europe.