My band is finally working up Dark Star so I'm working on both the guitar parts and the vocals. To get as fully as I can into the lyrics, one thing I did was I re-read The Annotated Grateful Dead Lyrics. That's where I saw:
Cirlot has this to say about the symbolism of the diamond:
"Eytmologically, it comes from the Sanskrit dyu, meaning 'luminous being.' It is a symbol of light and of brilliance. The word 'adamantine' is connected with the Greek adamas, meaning 'unconquerable.' In emblems, it often indicates the irradiant, mystic 'Centre'. Like all precious stones, it partakes of the general symbolism of treasures and riches, that is, moral and intellectual knowledge." (p. 81)
So Adamas graphite picks, which share the element carbon with their crystaline counterpart, the diamond, also share the Greek etymological root, adamas.
I tip my top hat to whoever chose the name.