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)?