In some sphalerites, tiny primary fluid inclusions can be distributed uniformly inside the stone or they can in some areas in its interior. These localized inclusions can mark some stages of the growth of the crystal, and can affect its transparency, from low turbidity to a translucent material.
Sphalerites virtually inclusions free are extremely rare; normally almost all faceted stones present some veils or fissures visible to the naked eye. The quantity of inclusions is extremely important for the valuation of sphalerites. Within the same color range, the price per carat for eye-clean stones is several times higher than for those with different quantities of inclusions.
Some examples of different clarity grades of Spanish gem sphalerites: