Canteras are Mexican opals in matrix. Some have features similar to agates, probably because they also form within cavities in matrix. These pictures show how the fire varies with angle of view. This opal appears to have goethite-coated rutile crystals in it, but I’m a novice for crystal identification. Many pictures like this are shown in my book, Canteras, Mexican Opals in Matrix. Slideshows are also at www.mudpot.com .
Jack Hobart