grinding materials on planets uses the same random spawn system we have for USS in supercruise - its random and inkonsistent.
materials spawning via mining laser from asteroids are not so random AFAIK.
finding the same asteroid again should yield the same materials again (never tested)
Not sure if it is even possible to find the same asteroid much less asteroid instance again, but you are correct - type and yields are not so random from asteroid mining.
In my OP results I did 3 mining sessions, each ~90-120min over couple days, then averaged the results I posted. While the numbers of each type varied somewhat, the types I got never did.
Forgot system name as I bookmark it but close to Diaguandri, has both pristine metallic and ice ring for normal vs ice mining, good station location for nearby resupply.
Same system, same planet, same A1 inner ring - always gets the same composition of raw mats I posted, just the numbers vary a bit but not wildly. Like at G1 I might get +/- 30-40 of one but then offset by lot more of another G1 type. But by G3 it never varies by more than just handful, like 15 vs 11, or 9 vs 14, etc.
And I can count on pretty much getting the same top two of G2 and G3 mats I always get: germanium / manganese --- molybdenum and niobium.
I basically need to find 3 more systems, each yielding the two other types of the G3 mats I'm looking for.
I'll try the close to star asteroid marker for G4 tip that someone above gave. Would really, really prefer to directly ship mine G4 raws vs trade up at 6:1 or 36:1
**Edit - also note, there doesn't seem to be much difference - none that I can tell - from mining for raws in pristine metallic vs metal rich. The system I used for my OP has two rings on the pristine gas planet, A1 pristine metallic, A2 metal rich. (and a nearby ice ring planet in same system - it's my 1 stop shop for normal 3P mining, raw mats mining, and ice mining (for synth resupply as yields of P, S, C, etc are far, far higher in ice rings but really bad for anything else)
The only thing that seems to matter for mats yields is using a prospector and low vs med/high mat yield asteroids.
low yield - don't even waste your time, you can fire the asteroid till fully depleted with a prospector limpet attached and not get ONE single G1 raw chunk. Low might as well mean non-existent. Sometimes get 1-2, sometimes none, usually just 1. Totally not even worth shooting as you fly by. Worthless.
med yield - always get some, usually 4-5 raw mats of some type if using prospector to enhance yield. Forgot exact numbers without prospector but not worth it.
high yield - best kind and worth it even without prospector enhancement, but yes yields of 8+ raw mat chunks by prospect first, then blast.
I haven't tried hirez instance but the time you spend on a single fight will likely negate any yield advantage. Lot faster/easier just sitting in a non-rez ring somewhere and constantly prospect.