Will come naturally over time, but you can prioritize certain things to unlock faster (buying all the decor upgrades that are 10-30 gems). Most of the upgrades that cost diamonds end up giving you more resources per run (and count towards the achievement) allowing you to purchase more house contract jobs, so these should absolutely be a priority.
Some things to keep in mind to help your gem/diamond hunting:
-Once you clear a run with a weapon, you’ll have to up the heat setting to keep receiving the big boss rewards (Titan blood, diamond, and ambrosia).
-Keep an eye out for the Poseidon boon that increases the amount of darkness/gems received
-occasionally chaos will offer a boon that increases the amount of gems/darkness received for the run as well
Depending on how lucky you get, and what you purchase this achievement will unlock eventually