Temperature of a Diamond in a Boiling Pot of Water Suppose I have a boiling pot of water (100 degrees C) and drop a diamond in. Does the diamond eventually reach 100 degrees C? 
Since the diamond is a rigid structure, its molecules do not vibrate that much. Water molecules colliding with the diamond may just bounce off elastically without imparting any velocity to the particles of the diamond. Because temperature is related to average speed, I think the diamond could remain at a lower temperature than the water indefinitely.
 A: Practically speaking, diamond has the highest thermal conductivity of any "reasonable" material, about 5 times greater than copper. Additionally, its specific heat is about 30% greater than copper or brass, but about 10% that of water. As a result, a diamond will heat up faster than just about anything else, particularly when immersed in a liquid like water with a high specific heat and which is boiling (produces strong convective currents to apply heat to the surface.
A: Note that to figure out how quickly an object heats up, you need the ratio of two quantities - the thermal conductivity, and the volumetric heat capacity. If the former is large AND the latter is small, you get an object that will quickly take up the heat of its surroundings.
This parameter is known as the "thermal diffusivity"
$$\alpha = \frac{k}{\rho c_p}$$
And it shows up in the thermal diffusion equation:
$$\frac{\partial T}{\partial t} = \alpha \nabla^2 T$$
Now the "stiffness" of diamond translates into a high bulk modulus: that is, a lot of force is needed to create a small displacement. For a sound wave, this means that the speed of sound will be very high since it is given by
$$c = \sqrt{\frac{K}{\rho}}$$
The higher the bulk modulus K, the higher the speed.
Finally, if you consider that thermal transport is really the transport of phonons through the solid, then clearly a higher modulus will result in better heat transport.
Putting it all together, diamond (with high speed of sound, low density, and low heat capacity) is an excellent heat conductor - and will quickly approach the temperature of its surroundings. Mathematically it will never "get there" since the solution to the heat diffusion equation is asymptotic. But since the cup of boiling water will itself cool down, there will actually be a time when the diamond is hotter than the surroundings. Which means there must be a point where the (mean) temperature of the diamond is equal to the (mean) temperature of the water.
