Rainbow Calsilica | Everything There is to Know

The Story of Rainbow Calsilica

Rainbow calsilica is a man-made material audaciously introduced at the Tucson Gem and Mineral Show 2002. It attracted much attention, but many were suspicious about this curious-looking stone. 

The world's only supplier claimed he imported rainbow calsilica in large slabs from Chihuahua, Mexico. He insisted it was a natural rock, but experts were far from convinced.

Those behind the production of rainbow calsilica rented a large corporate trade stand decorated with high-quality photographs of what was claimed to be the mine in Mexico. Pictures of the stone being mined were also on display, and hundreds of glossy catalogues were given out every day. 

A huge amount of time, money, and effort had been invested to convince the world that rainbow calsilica was a natural stone.

When geologists asked to inspect the mine, they were repeatedly denied. They were told the landowners wanted to protect rainbow calsilica from exploitation, so could not share its precise location.

The Scientific Analysis

Having tested rainbow calsilica purchased during the show, some believed it was a cryptocrystalline calcite with clay minerals acting as bonding agents. Another group of geologists confirmed it was man-made with synthetic colours.

The supplier of rainbow calsilica claimed it was stabilised with an epoxy to increase durability. He said that is what's likely to be showing up in tests.

Rocks and minerals are often stabilised to prevent erosion. The process involves filling holes or damaged sections with resin or other substances. This can be used to repair some gem-grade material before being cut and polished.  

Despite growing speculation over the authenticity of rainbow calsilica, access to the mine continued to be denied. The story about wanting to protect it from exploitation didn't ring true.

If rainbow calsilica were a natural stone, it would have been an extraordinary discovery. Such a find would lead to extensive publicity and a dramatic increase in revenue. With the mine being on private land as they claimed, access could be easily controlled.

As the supply of rainbow calsilica increased, it began appearing at mineral fairs worldwide. In the meantime, collectors and enthusiasts became more determined to establish what it really was.

Having inspected several samples, one group concluded it was made primarily of crushed calcium carbonate. Calcium carbonate is commonly found in rocks as calcite and aragonite.

It had then been coloured using artificial dyes. These included PB15 which is a blue pigment, and PY1 which is a greenish yellow pigment also known as hansa yellow.  Plastic-like stabilisers had also been used, one of which included traces of a paraffin derivative. Other chemicals which could not be clearly identified were also present.

Traces of hematite, celestine and calcite were also identified. Many of the particles had seemingly been bonded together using a soft plastic-like substance similar to paraffin wax.

Little is known about those behind the production of rainbow calsilica. Presenting it as a natural stone at the Tucson Rock and Mineral show was audacious, but this geological hoax had finally been exposed.

Rainbow calsilica continues to be used for decorative purposes. It's sometimes confused for Fordite, but the two materials are not the same.

Reference:  Kiefert (2003) Rainbow Calsilica, The Journal of the Gemmological Association of Hong Kong 24: 41-46.

Article Pictures

The rainbow calsilica in the second photo is from our collection.

The third photo is courtesy of James St.John. The image is clickable and redirects to the original photo. 

Pop-up photos: Aragonite - Courtesy of Stan Celestian. Celestine - Courtesy of Ron Wolf.