Zircon is a natural mineral, while cubic zirconia is a synthetic material created specifically as a diamond simulant. They both serve as diamond substitutes, but they aren’t the same substance. Zircon’s crystal structure is tetragonal and it exhibits birefringence, which can cause faint doubled images when viewed through a loupe. Cubic zirconia, on the other hand, is cubic and isotropic, so it does not show birefringence. Differences in refractive index, density, and hardness also separate them: CZ is generally harder and denser than natural zircon, and its fire comes from its very high refractive index and dispersion. Because of these distinct properties, they are considered different simulants.

Earth has distinct layers, and the term for the outermost solid shell that forms the surface you walk on is the crust. This layer sits above the mantle and comes in two main types: continental crust, which is thicker and lighter in composition, and oceanic crust, which is thinner and more mafic. The crust is literally the surface and the outermost layer of the planet’s solid structure. The core refers to the center of Earth, cratons are ancient, stable parts of the crust and upper mantle, and an eruption is a volcanic event, not a layer of the planet.

The key idea is how visible clarity characteristics are in a diamond. Clarity grade is driven by how noticeable inclusions or blemishes are, which depends on three factors: the size of the characteristic, where it’s located inside the stone, and how much it stands out (its relief) against the surrounding facets and background. These determine how easily the imperfection is seen under magnification or by the eye, and thus how the clarity grade is assigned. Weight, or carat, measures the overall size of the gem but does not change how clearly a specific inclusion or blemish is seen. So it doesn’t influence the clarity grade, making it the factor that isn’t listed among those that affect clarity characteristics’ influence on the grade.

The table percentage expresses how large the diamond’s table is relative to the stone’s overall width. The table is the flat facet on the very top, and its width is measured across the face, then divided by the diamond’s average girdle diameter and multiplied by 100 to give a percentage. Using the average girdle diameter, rather than a single girdle measurement, accounts for any slight asymmetry and gives a consistent reference across stones. This value tells you how big the top facet appears compared with the stone’s width, which influences light performance and appearance. So the correct concept is that the table size is expressed as a percentage of the average girdle diameter. The other ideas mix up what is being compared: it’s not the table depth, crown height, or a percentage of a single girdle dimension.