I have spent all of the time in OC so far, I haven't done any outside FATEs.

I have completed 111 FATEs and 64 CEs, 175 events in total.

In that time, I have acquired:
1 x Verdigris,
1 x Azurite
2 x Malachite
2 x Realgar
2 x Orpiment
2 x Caput Mortuum

That's a total of 10 Demiatma so far.

The drop rate here for me, based on the above is 17.5%.

That's not a bad drop rate at all, in my opinion.
I'm not actively grinding for the relic, it's a passive grind whilst I level the Phantom jobs.