Shortly after he began work on tektites in 1960, Chao was given a sample of sandstone from the vicinity of Meteor Crater, Arizona.
From this material, he was able to isolate an unusual mineral with high refractive index, which he showed to be a high-pressure polymorph of silicon dioxide.
[4] Coesite and stishovite became known as hallmarks of impact crater events, which were essentially the only natural processes that produced high enough pressures to transform ordinary quartz into both of these dense minerals.
Chao made many pioneering studies on tektites, and discovered the occurrence of iron-nickel metal in specimens from the Philippines.
His work on Moon rocks included being a member of preliminary examination teams and a principal investigator during the Apollo 11-17 research programs.