Spectrophotometric study of the colour of pearl has been made on the shell of cultured pearl by removing its seed.
The only effective pigment in pearl layer is of orange colour. If this pigmentation is dence, the pearl appears yellow, and if light, the colouring is not affected.
So-called body colour of pearl is determined by the structure thus:
1. It is blueish if brownish black organic matter is interlaid between the shell and the seed.
2. If air is present underneath the shell, total reflection of light in the layer makes the pearl appear silvery.
3. Coarse crystalline structure of the layer gives white colouring.
So-called surface colour of pearl is by scattering and surface reflection of light. The scattering is caused by two kinds of layer, one rich in organic matter and the other rich in inorganic matter, lying one on top of the other with uneven boundary.
Considering that the surface colouring is due to interference of polarized light, the degree of polarization of reflected light is investigated in relation to the colouring.
The pearl layer appears as optically monoaxial with its axis perpendicular to the surface.
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