FRANK R. PAUL'S INHABITANTS OF IO / IONIANS

 In 1940, science-fiction art pionneer Frank R. Paul did pieces showing what life could be on other planets. In his own words:

Theoretically, creatures such as these might inhabit this satellite. Io has a dimater of 2000 miles and a thin, though breathable atmosphere....  an entire city built of gleaming, jewel-like crystals. This is what a visitor will find in the weirdly beautiful Crystallis, the capital city of Io, satellite of Jupiter.

Source: Frank R. Paul, Amazing Stories.