Black stone (weapon) can be obtain in Rafinery level 3:
200 Black stone powder + 3 Tyrant;s Blood + 2 Oil of Fortitude

There is no processing choices as Grinding for Black Gems nor Alchemy Stone Shards+Black Stone powder

Only Sharp Black Crystal Shard can be turned into Black Stones

Processing Master 26:
From 200x Sharp Black Crystal Shards (100 cycles)
I got 234x Black Stones

200 SBCS = 762 mil silver
234 BS = 30 mil
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Income = -731 mil