Yes, this is a fragment of a real dinosaur eggshell! It comes from Aix-en-Provence, France, and is attributed to the titanosaur sauropod species Hypselosaurus priscus, although this is not definite. This species may have been around 40' (12 metres) long. These eggshells are abnormally thin, which makes them work well in miniature settings. The preservation of the shell structure is incredible - you can see the texture and how porous the shell would have been.
3/8 inch (0.95 cm) wide, just under 1/4 inch (0.6 cm) deep, 5/8 inch (1.6 cm) tall.