The upward force of the chain on the diamond would be the tension in the chain, and this tension would have to support the weight of the 45g that hangs from the chain.
mass = 45 g = 45/1000 kg = 0.045kg
Weight = mg = 0.045 * 10 ≈ 0.45N, g ≈ 10 m/s²
So the upward force is ≈ 0.45N.