The correct answer is D. Diamond is lustrous because its refractive index is high.
Refractive index is a measure of how much light bends when it passes from one medium to another. Diamonds have a very high refractive index, which means that light bends a lot when it passes through them. This is what gives diamonds their characteristic sparkle.
The other options are incorrect. Option A is incorrect because diamonds can be found in a variety of colors, including white, yellow, brown, and black. Option B is incorrect because there are many other materials that are harder than diamond, such as cubic boron nitride. Option C is incorrect because diamonds are not always pure. In fact, many diamonds contain impurities that can affect their color and clarity.