Mother-of-pearl is the same dull chalk an abalone builds its shell from — aragonite, stacked in hundreds of tile-thin plates about 500 nm each. No dye lives here. The color is in the stacking: every plate boundary reflects a sliver of light, and the slivers interfere. Tilt the shell and the sheen sweeps, because you changed the path length through the stack.
Drag to tilt the shell — the pearl sheen sweeps across it. Release and it drifts on its own.