The amount of ores is also relevant in your skill level. No one wants to get half an ore per rock, but everyone wants a stack of five. How this works is as follows:
You are able to obtain one more stack for every time your current skill level reaches a multiplier for that rock. Such as a rock having 20 skill to mine, once you reach 40 skill you will have a higher chance. Similarly to a rock having 15 skill to mine, you able to obtain more if you reach 30 skill, 45, and so forth.
Once that has been determined you will always receive one ore and a chance of 50/n where n=the stacks you are bound to get.100% for single stack