Egg cups are usually made of ceramic, metal, wood, glass or plastic, but they can be made with anything else. Or at least that's what we're obliged to believe after seeing what we've seen. Sometimes the material used can be so rare or so beautiful that no extra decoration is needed - that's what happens most of the time with all the different varieties or marble, for instance. But it can also happen that the unusual material is used in order to allow detailed decoration - that concerns papier maché, used because it's easier to produce perfectly looking painted pieces with papier maché than with wood.
Known details: 1) includes fake grass [eggcup2233]; 2) soapstone, made in Kenya [eggcup0670]; 3) marble, made in Pakistan [eggcup3349]; 4) includes water, "Happy Flowers" series by Culto (Germany) [eggcup4647]; 5) elastomer, "Baby" egg cup designed by Fred Rieffel for Pylones (France) [eggcup1430]; 6) silicon, "Weich-ei" egg cup designed by Helena Leba (Germany), Softies collection [eggcup2968]; papier maché, "made in USSR" [eggcup0850].
If you like to see some impossible and virtual egg cups, please check refer to The Ultimate Egg Cup Resources Guide post.
If you want to see egg cups in more usual materials, you could check the Porcelain egg cups category, the Wooden egg cups page, the Glass egg cups selection, the Plastic egg cups introduction or, of course, all of them.