The calcite crystals line up perpendicular to the shell's surface, giving it a coarsely fibrous appearance. Inside is the nacreous layer. When a bit of grit gets between the mollusk's tissues and nacreous layer, the mollusk secretes a coating of nacre around it, isolating it from the rest of the shell. Blister pearls, growths still attached partly to the shell, show an intermediary stage of this process. The answer is linked to the common test used to distinguish true from artificial pearls.
True pearls feel rough against the teeth.
This is because the surface of the nacre consists of thousands of tiny overlapping plates of aragonite, whose edges give the rough sensation. These edges also interact with the light that falls on the pearl's surface. The light striking the pearl experiences a phenomenon called diffraction in which light is split up into a spectrum of color by passing through a tiny opening. The different colors of light bend differently, separating them into their color. The edges of the aragonite flakes act as tiny diffraction gratings, and the colors we see are tiny spectra, playing out as we turn the pearl to different angles.