theory goes that Aechmea kept Craig's left arm from ch67 (and substituted it with another when reconstructing him) to later manipulate him into thinking that he's always loved him specifically and wouldn't do this to just any gem
All that he needs to know is that>Cairo lost his old left arm 102 years ago during Ghost's last battle
Which he has all reasons to know and is proven by his rightly identifying him as 'that double-layered quartz'>and the arm is a symbol of his identity issues
Which he knows because it's snapped whole during ch67 and he could easily make 2+2 from the whole analysis he made about Ghost like a minute earlier
Plus Karen definitely told him about those issues like in pic related so Aechmea had more reasons to believe that returning it could be seen as a way to make her feel whole and free
Except Karen doesn't see it the same way because the arm to her is a symbol of her old attachments (to Ghost, to Phos, to Lapis) and rightfully so.
Also, let's consider the other possible explanations and why the grinding scene changes its meaning significantly if those were true or is at least less powerful>the arm is just fake
Then Karen isn't grinding anything real and Aechmea is just a poor counterfeiter with no deeper scheming or reasoning to his actions>the arm is the real one from 102 years ago
Then Aechmea's actions make little sense, seeing as he never did anything during those years, he wouldn't have any reason to fall in love with a random gem, and didn't show interest in Craig until he went to him.