They are metal eggs decorated very fancily with gold, silver, and/or gemstones.
Role in series
"Rule of Dumb"
A cabinet filled with ornamental eggs appears in Patrick's house.
"What Ever Happened to SpongeBob?"
Squidward has a collection of these eggs. When he does not want to go to New Kelp City to get SpongeBob, Mr. Krabs offers one of his ornamental eggs to Squidward if he can bring him back. Squidward agrees, but only because it will complete his collection, and therefore leads everyone else to New Kelp City.
After bringing SpongeBob back, Mr. Krabs gives the egg to Squidward, but he immediately slips, loses the egg, and it breaks on SpongeBob's head, which restores his memory.