A large colonial ascidian, forming globular colonies. Smaller colonies are spherical, larger ones squat, with a small basal attachment. The test is transparent, with a greenish tinge when out of water. The zooids are large, with white markings similar to Clavelina lepadiformis. This marking consists of a white line along the endostyle, dorsal lamina and around the top of the branchial sac, and six small white spots around the atrial siphon. 50-400mm diameter colonies. Only Clavelina lepadiformis has similarly marked zooids, but that species does not have a massive common test, the zooids are only joined at their bases. Diazona is larger and the six small white spots around the atrial siphon are diagnostic.