Multicellular castings were construction materiel used to manufacture the artifacts that appeared on the Shore Leave Planet.
In 2267, a USS Enterprise landing party encountered several of these artifacts. Spock, after analyzing them with his tricorder, learned that their structure resembled the cellular casting used for wound repairs by 23rd century physicians, although the material composing the artifacts was "much finer". Studying the Black Knight, Spock realized that everything on the planet had the same basic cell structure – leading him to conclude that all of it was manufactured. This conclusion was later confirmed by the Keeper. (TOS: "Shore Leave")