A few suggestions:
Find a hotspot you want to mine. The closer it is to the star, the brighter it will be. Try to use EDDB to find one close by your location, with pristine resources, and say, >5000 ls from the star.
Once you arrive at the hotspot, select the star in the left panel, and always fly towards the star while scanning the rocks. That makes the orange ones easier to spot. If you also enable night vision, you will find it easier to spot if the rock has fissures, without firing a prospector limpet.
Finally, don't forget to do a 180 once in a while, because
the game mechanics often cause rocks with cores to spawn behind you.
Good luck.