porcelain (n.)

ceramic ware having a translucent body, and, when it is glazed, a translucent glaze, 1530s, from French porcelaine and directly from Italian porcellana "porcelain" (13c.), literally "cowrie shell;" the chinaware being so called from resemblance of its lustrous transparency to the shiny surface of the shells. As an adjective from 1590s.

porcelain is china & china is p.; there is no recondite difference between the two things, which indeed are not two, but one; & the difference between the two words is merely that china is the homely term, while porcelain is exotic & literary. [Fowler]

The shell's name in Italian is from porcella "young sow," fem. of Latin porcellus "young pig," diminutive of porculus "piglet," itself a diminutive of porcus "pig" (from PIE root *porko- "young pig"). Compare Greek khorinē "cowrie," also literally "a little pig." 

According to an old theory, the connection of the shell and the pig is a perceived resemblance of the shell (also Venus shell) opening to the exposed outer genitalia of pigs. For a different answer, Century Dictionary (1897) writes that the shell was "so called because the shape of the upper surface resembles the curve of a pig's back."

Thompson ["A Glossary of Greek Fishes"] writes, "According to a widespread belief the cowries were a charm or talisman against sterility. They are among the women's ornaments at Pompeii, and are found in women's graves in France and England as late as the Middle Age ...." He writes that the bigger, showier shells, from the Red Sea, probably account for the "popular and erotic names," but adds that the association of the shells with female sex is almost worldwide.