Diamonds themselves cannot be made in a laboratory very well, but crystals with similar properties and appearances can be made. One popular type of “simulated diamond” is a cubic zirconia crystal, which is a crystal with the chemical formula ZrO₂. Cubic zirconia are more dense than diamonds, or, in other words, a cubic zirconia is heavier than a diamond of the same size. They are also a fairly hard material as minerals go, but they are not as hard as diamonds and can be scratched by diamonds. Cubic zirconia are usually colorless, which isn’t the case for diamonds. Most diamonds contain some amount of noncarbon impurity that makes them appear colored, and only very pure diamonds appear colorless.