The Book of Ancient Secrets Eldritch Invocation says that a Warlock can add ritual spells to his book of Shadows, and that this requires 50 gp worth of rare inks per spell level.
A wizard can copy additional spells to her spellbook which costs 50 gp per spell level which represents "the material components you expended as you experiment with the spell to master it, as well as fine inks you need to record it."
Is there any indication as to whether these inks are the same? (If the Warlock runs out of ink, can the Wizard share some of hers, and would that work?)
Best Answer
The published rules don't go into this level of detail. As far as I see it there are two basic ways to rule this.
The pragmatic approach
Fine inks are rare, and rare inks are fine. They cost about the same, so they are the same.
The flavourful approach
For example: A wizard's fine inks are the sort you could use to write an invitation to a society ball. They are expensive and durable. A warlock's rare inks dripped from some eldrich rift in reality and are unsuitable for any other purpose than recording a warlock's spells.