Carbon-rich Planet's Core and Mantle structure

http://www.universetoday.com/91596/carbon-super-earths-diamond-planets/

Basically, a carbon-rich planet can have diamond layers in the mantle.
But never thought of a carbon-rich + iron-rich = a planet's core made of steel or at least a layer of steel.

Also, due to diamond's property of tranfer heat efficiently.
A mantle that has veins or thick layers of diamonds or connecting diamonds mean that the planet may be very cool.
As in a cooler mantle and little to no plate techtonics.
As for the atmosphere recycling gas, may cause the atmosphere to be thinner.