The nacre, which is the substance which is composed of microscopic plates of aragonite (calcium carbonate) cemented together with conchiolin, is from what mother-of-pearl is made. It is also the substance an oyster deposits, layer by layer, around alien matter to form a pearl.

A thin nacre that allows the nucleus to shine through shows that the pearl was only in the mussel for a short time. It is not nice nor solid because a thin nacre will soon wear off. The thicker the nacre, the better and more valuable the pearl.

An exact definition of the thickness of the nacre can only be made at a specialized laboratory using X-ray.