A freshwater pearl is nestled in the aperture of a green sea shell and then displayed under a glass dome. The dome is glued in place. The shell is pierced with two small holes but the one visible from the front is in a location where a number of shells naturally have a hole called an umbilicus, so I'm not sure if this hole is natural or drilled. The small hole visible from the back is definitely drilled (third photo).
1 inch (2.5 cm) diameter and 1 1/8 inches (2.9 cm) tall.