Some of the very finest material comes from the San Carlos Apache Reservation which is also famous for the peridot that has long been mined there. It usually forms as a botryoidal chalcedony, that is, a bubbly appearing semi transparent quartz. The colors in fire agate are caused by the deposition of many very thin layers of iron oxide (probably goethite) on the chalcedony bubbles which were then overgrown with more layers of chalcedony and more iron oxide. The colors one sees are actually produced by the interference of light caused by these very thin layers, something like the effect one sees with oil on water.
There can be quite a range of colors observed in fire agate as well as a fascinating variety of color patterns. You can see bubbles, big, small, and tiny, single or many. You can have broad flashes of color, multiple bull's eyes, almost anything you can imagine. Many stones have a sculptural, three-dimensional quality to them, which is maximized by skillful lapidary treatment. It can be a very challenging stone to cut and polish in a manner that reveals its maximum beauty because the cutter must follow the curves and dips of the iron oxide layers, getting as close as possible to them for best color, but being very careful not to cut thru the layer and leave a spot with no color. Due to the way fire agate forms it is seldom possible, or desirable, to cut standardized, calibrated stone. Instead one must follow the contours established as the stone grew. It requires patience, perseverance, and imagination.
Fire agate can show almost any color. Orange is the most common, with green and purple close behind. Red and blue are the least common. Often the apparent colors are produced by combination of several colors. When all three primary colors, as well as additional secondary ones, are displayed vividly you have the most valuable of stones, sometimes called a "peacock". Although exceptional examples can bring astonishing prices, from hundreds to thousands of dollars for the most desirable stones, it is also quite possible to find lovely cabs at very affordable prices.