Rainbow Calsilica (or Calsilica) is an opaque gem with colourful stripes/layers of light and dark blue, green, yellow, orange, red, black and occasionally even pink. Calsilica is a popular gem and is readily available as cabochons and carvings.
It has been claimed that Calsilica is a natural form of microcrystalline Calcite with layers of various natural clay minerals as the bonding and coloring agents. It is claimed to be mined in Chihuahua, Mexico were it was allegedly discovered in the veins or seams of volcanic rhyolite in the mine. Although suppliers claim that Rainbow Calsilica is a natural stone, tests have indicated that it is powdered carbonate rock artificially coloured and then stabilized with plastic. It contains a mixture of Calcite, silica, plastic and artifical colouring pigments such as PB15, an art-supply pigment, and a paraffin-like material. [ref: Winter 2002, Gems and Gemology, journal of the Gemological Institute of America].
In 2003, the Swiss Gemological Institute published an article on Calsilica in their newsletter. SSEF purchased two specimens at a mineral show in France in 2002. The seller produced photographs of the alleged mine in Mexico along with an authentication letter from a US laboratory stating that the materials were not man-made. SSEF’s testing found that the base white materials were indeed Calcite. However, they also found man-made colouring pigments within the specimens. and that the particles were bonded together by a transparent soft resin material that was very similar to paraffin wax and then hardened making it suitable for the jewellery market.