Summarize:

Allow the shells to dry for at least one full day and then rub oil on the shell.  Mineral oil not only restores the shine of the shell, but helps to preserve the shell. Similarly, you can use WD-40. However, when using this material, be sure to use gloves to handle the shells. You can use satin-finish polyurethane or coat with clear nail polish. This type of finish preserves the shell's organic look while giving it an extra glossy shine. Do one side of the shell each day. Allow the shell to dry completely before starting on the other side. Each side can take about a day to dry.
Rub mineral oil over each shell to give it a deep gleam. Spray the shells.