After spending just over 4 days in Attowah just trying to farm this item for me and my ls mate, it was the only quest we had left to do for Saviour Atma, we decided to post on the forums as we didnt see one drop, one of those 4 day we actually spent 19hours in attowah none stop with capped amber etc so you can imagine the frustration. We made in excess of 20 GM calls between us to see if it had been broken in an update or something but no help was forthcoming.
Then we had a reply on the forums from a guy who said he farmed these regularly from the black mandies, and that the malachite is actually considered a junk item so capping amber is probably a bad idea. Well after being all out of any other ideas we jokingly set off to try this. We killed 4 mobs with amber kills and then just worked on azure and pearl to keep chests and time ticking over.
Then the impossible happened, only 40 mins in we got our first, 4 hours later we got our second! so i would seriously try this if you are experiencing the same luck myself and my ls mate were.
Good luck!