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Rainbow Calsilica Everything Worth Knowing

manmade rainbow calsilica stone. A close up photo showing the individual coloured layers

The Curious Tale of Rainbow Calsilica

Rainbow calsilica is a man-made material audaciously introduced at the Tucson Gem and Mineral Show in 2002. It attracted plenty of 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 in Mexico.  He insisted it was a natural rock but experts were not 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.  There were also photographs of the stone being mined.  Hundreds of glossy catalogues with information and photos 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, their request was repeatedly denied.  They were told the owners of the land wanted to protect rainbow calsilica from exploitation.  As a result they were unable to share its precise location.

rainbow calsilica stone

The Scientific Analysis

When tests were conducted on 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 the stone 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 it's 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 was 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.

section of the manmade stone rainbow calsilica

As the supply of rainbow calsilica increased it began appearing at mineral fairs around the world.  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.  The third image courtesy of James St.John (Flickr).

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