The Queretaro area, 200 km northwest of Mexico City, has been producing fine, unusually transparent opal with vivid red and green play of color for over 100 years. These opals occur largely in gas cavities in pink to brick-red, thinly bedded rhyolitic lava flows and are mined in open-pit quarries. After examining thousands of opals from this area, the authors selected 20 specimens containing excellent examples of both common and rare inclusions that seemed to represent the variety of inclusions found in gem-quality opals from this locality. The opals and their inclusions were then subjected to a battery of tests to ascertain their nature. These tests revealed both two- and three-phase inclusions and a variety of different mineral species.