Hi Nick

Diamond has a density of 3.52. It's a lot stiffer than iron/silicate mixes so there's not a huge amount of compression for smaller objects. Denser liquid diamond is likely at 6000 K and a few megabar.

Compression means a terrestrial planet's radius is related to the mass by an index of 0.265-0.272 rather than 0.333.

Straight ice goes through a lot of ice phase changes with geological scale pressures. Starting at 0.917 at STP it can climb with pressure then change into Ice VII at 22,000 bar plus direct from liquid water at 355 K. Past that the density keeps climbing. And 22,000 bar is just 220 km of water at 1 gee. Fine for estimating moon sizes, but tricky for Earth masses.

Adam