The best I have found so far is in Dresdan. To the right of the first station (Forget the name off hand) is a diamond field. Become friends with the hessians and the local miners. You can sit there all day without worrying about attack.

Plus...this is the place you can pick up a Sabre so its a good home base if you don't want to go with the lemmings..err I mean Corsairs

The asteroid field behind Bautzen station in Dresden is easier for mining diamonds than in Omega-11, where there's some pretty heavy radiation in the mineable areas. Also in Dresden is Leipzig Station, which buys Niobium for $1600, only a little less than the $1650 you can get for diamonds on New Tokyo. I've only found one area to mine Niobium, in Tau-23, which has an Outcast base. BTW, if you look at the map with the mineable areas highlighting turned on, the brighter the yellow color is, the more "rich" the field is; meaning you get something more often when you shoot a rock or debris. Sometimes there are areas inside a field that are richer than others, so it's much faster (and less boring) to do the mining there.

Thanx