I was very lucky with atma drops in FATEs but I have yet to get an Alexandrite from them. I tried for a couple hours in a few areas on the first few days Novous was released and then gave up because it felt like I was wasting my time. The areas I tried and failed at are: Camp Bronze Lake, Camp Overlook, Summerford Farms, Aleport, Camp Bluefrog/Processing Plant and Dragonhead. Every minute I didn't get a drop from a FATE I could have been working towards grinding myth for an Alexandrite.

Just providing some information. I think Alexandrite's are exactly the same as atma's; strictly RNG.