The Omni Geode is a Mineral deposit found primarily by breaking rocks in The Mines. It must be broken open at the Blacksmith for 25g to obtain the items inside. Omni Geodes can contain minerals, artifacts, ores, or basic resources.
I farmed 500 Omni Geodes in the Skull Cavern and had them processed at Clint's. The yield wasn't quite as expected (only 1 of this precious gem), but more than sufficient to donate to Gunther's museum and to build a Mineral Museum on my own farm.
What is inside 500 Omni Geodes?
These are the results of my experiment with version 1.5.4 on PlayStation 4 of February 2021: