How do I obtain diamonds in Hades?

The House Contractor says I need 1 diamond to free Orpheus from solitary confinement. I haven't run into any diamonds in any runs I've done, just obols and darkness and nectar and gemstones. I've only beat Megara once, so maybe it's something in Asphodel I haven't reached yet, but I'm not sure.

How do I get diamonds? Is it just that I've not progressed far enough yet, or do I have to unlock something else first (like some of the other house bits or what have you)?

Solution 1:

You get a diamond for defeating the Bone Hydra and escaping Asphodel, the second biome of Hades. This is the primary way to get diamonds

You can also get diamonds from the Fated List of Minor Prophecies and from the Wretched Broker, though I think the broker trade is unlocked by defeating the Bone Hydra anyway.