They are metal eggs decorated very fancily with gold, silver, and/or diamonds.
Squidward has a collection of these eggs, as shown in "What Ever Happened to SpongeBob?" When Squidward did not want to go to New Kelp City to get SpongeBob, Mr. Krabs offered one of his Ornamental Eggs to Squidward if he could 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 it later breaks on SpongeBob's head which helped regained his memory.
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