[elihu]

The density of diamond is about 3.5 g/cm^3. The Earth's moon has a density of around 3.3 g/cm^3, so if you replaced it with a diamond of about the same size, it actually wouldn't be all that different in terms of gravity. Solar eclipses would be pretty wild though.

[djtango]

Was more thinking about whether there are any interesting gravitational effects on the internal chemistry of a giant crystal. Say it is one single crystal, how would its centre differ from the surface?

[defrost]

Earth, Moon, or Big Fine Diamond, the gravity at the surface is 1 unit (for whatever normalised radius, density to get a 1 works) while the gravity at the centre is 0.