Shrinkage Cracks in Fire Agate
Over the last few years, the author has encountered numerous examples of fire agate (a variety of quartz chalcedony) displaying a random weblike pattern, including a parcel of cabochons from Mexico purchased at the 2023 Tucson gem shows. Fire agates typically show a botryoidal structure reminiscent of a cluster of grapes, so this unusual pattern was quite noteworthy.
Microscopic examination of the cabochons revealed that the pattern was most likely the result of shrinkage cracks caused by desiccation. This drying out created a volume reduction that pulled apart the silica material during formation of the fire agate, producing a pattern of irregular separations that were later infilled by a secondary generation of chalcedony, which then preserved the pattern (see above). These shrinkage cracks stand out in high contrast to the host chalcedony as interruptions in the iridescent phenomenon that result from broken layers of goethite where shrinkage occurred. While not particularly common, shrinkage cracks in fire agate create a unique pattern in this colorful gem material.
