I believe once you rank up you have a new mine that allows for a larger abundance in ores that you will be selling more often than the previous mine. If the ratio of emeralds to diamonds is the same then this is possibly just a wall you've hit that you need to overcome. Usually in prison game modes each mine has a larger amount of something, but sometimes the patterns are changed and you have to work more with the same things in order to move to the next level.
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