The fifth Ruby being called "Doc" by Steven is likely a reference to the seven dwarfs. Six of them were named for feelings (Bashful, Grumpy, Happy, Dopey, Sneezy and Sleepy) and the seventh was called Doc, breaking the pattern. Likewise, four of the Rubies are named for where their gem is located (Army, Eyeball, Leggy and Navy) and the fifth is Doc.