Fire Agate is a layered stone. The layers are small enough that light entering them forms interference colors known as “fire”. The gem is thought to be formed when hot water saturated with colloidal silica and iron oxide invades cavities in country rock and begin to cool. Chalcedony with iron oxide begins to grow on any available surface (the iron oxide gives the basic brown color to the gem). As the solutions began to precipitate and grow layers of silica and iron oxide would be deposited depending on the relative level of those elements in solution and underlying conditions. These alternating silica and iron oxide layers (Schiller layers) cause the brilliant fire in the gem. As iron oxide ran out in the solution colorless chalcedony continued to grow.
Many of these gems are found in the land between Kingman, Arizona and Needles, California, and around the Colorado River. Fire Agate has also been found in quantity in some areas of Mexico.
Working with Fire Agate
Cutting Fire Agate essentially reverses nature’s process by grinding and polishing away layers, following natural contours, until only the fire is visible. The most common colors are reds, greens and oranges. Deep purples and blues are rare and generally demand higher prices.
A rare variety of Fire Agate is Sagenitic Fire Agate, The main difference being that the stone is cut to showcase an uncommon Spray of Needle-like inclusions, often these needles will show a similar “fire” to stones that showcase the botroydal structure.