46.35 ct. Montana Rainbow 'Iris' Agate
True Rainbow 'Iris' Agate is a rather astounding gem material that is quite rare in the market. In normal reflected light, the stones look like simple Agate slices, however, once transmitted light is applied, they come alive with a full spectral display of color. The finest material comes from the Yellowstone River in eastern Montana - even when a piece is found, however, it takes an experienced polisher to bring out the colors. As the effect is caused by very thin bands within the Agate that diffract the light, the slice must be oriented in the exactly correct direction and cut to the correct thickness. Any mis-step and the stone will not show the rainbow colors.
The quality of pieces varies widely - often there is
just one thin band that 'rainbows' - here you get a lovely zone that occupies
nearly the entire bottom half of the stone. What is more, the colors
change dramatically as a light source is moved - see the above, right image vs.
the below. A great overall look, this is a scarce piece of a
phenomenal US gem material.