Yes, it's ebony.
They are Irish 5p coins. The side with the bull is the volume, and the side with the harp is tone.
Backstory, Quicksilver Messenger Service got their name because there were four Virgos in the band. Virgos are ruled by the planet Mercury, Quicksilver is another name for the element Mercury. On John Cipollina's guitar, there are Mercury dimes on the control knobs. The chance of Brian finding four silver Mercury dimes in Ireland in excellent condition was out of the question for the most part. Irish 5p coins are the same size of U.S. ten cent pieces, and Brian thought the the bull and the harp would be perfect in the irony of it's use, and that the guitar was made in Ireland. Pretty cool all the connections, eh?
View attachment 50641