If the mother-of-pearl shell surface is not permanently damaged, the loss of shine may be due to the material being soiled. Mother-of-pearl shell is a porous material and care should be taken when cleaning as this can damage the outer layers and contribute to permanent loss of shine. As a rule, any cleaning technique that is safe for pearls will be safe for your mother-of-pearl shell.
Avoid abrasive materials, as they will damage the outer layers and finish. Jewel Brite™ jewelry cleaner includes a very soft brush, contains no ammonia, acid or alcohol and is generally safe for pearls and mother-of-pearl shell.
Test a small area first for any adverse reactions
After cleaning, dry the beads gently with a soft cloth
After the beads are clean and dry, you can apply a light coating of mineral oil with a Q-tip, let sit overnight and wipe away any excess oil with a soft cloth.
If the mother-of-pearl shell surface is permanently damaged and the cleaning process does not restore the shine, you can consider (as a last resort) using Magic-Glos® top coat to add a surface shine. After applying Magic Glos per manufactures instructions, move the design to a source of UV light--this can be a true UV light source or somewhere in direct sunlight. The curing time will vary depending on the intensity of the light.
Note: Sealing mother-of-pearl shell is generally not recommended as it can result in a reduced luster or worst-case, potentially cracking the surface.