Okay so I went out and found an Alexandrite hotspot and I am still having trouble finding the right rocks, but I have 29 Alexandrite, 14, Void Opals, and 16 Grandidierite after about 45mins of searching random rocks trying to get the hang of recognizing the right ones.
Is that good, bad, or about right for a first timer?
If you're finding 'roids to blow up roughly once every 5-10 minutes, you're doing fine.
My SOP is to blow up every deep-core roid I find and collect everything.
Sooner or later I'll have to start dumping limpets.
At this point, I start to consider if the next roid I find is more valuable than what's already in my hold.
If, for example, my cargo hold is full and I've got 40 limpets and 40t of Alexandrite and I find a VO roid, I'll dump 15t of Alexandrite (or whatever's the lowest value ore in my hold) so I can collect the VO's.
Effectively, I hang on to limpets and dump low-value ore so I can replace it with ore of a higher value.
That way I can keep on mining for as long as possible, constantly replacing low-value ore with more profitable stuff.