Yuki, First I would recommend you loose the Dremel and get a good
flex shaft. That way the motor will be isolated from your work…both
dust and water.
Second, get a good diamond burr the size and shape you want. It
will work more easily on the shell with less vibration.
Third, use a liberal amount of water whilst drilling. If you can,
actually immerse the shell in water (hence the need for a flex shaft
so the water won’t get into the motor). Water will keep the drill
and shell cool, keep the drill hole clean and perhaps most
importantly, keep the dust out of your tender lungs!
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2