Yes, shellac (in spite of its conchological sounding name) does indeed ruin
the appearance of a shell.  It imparts an unnatural gloss, and as Carol said,
yellows over time.  A properly oiled shell will look natural, but a
shellacked shell never looks natural.  It just looks shellacked.  Shellac can
be easily removed with methyl alcohol, which is sold in paint stores as
shellac thinner.
Paul M.