1530s, from Middle French porcelaine and directly from Italian porcellana "porcelain" (13c.), literally "cowrie shell," the chinaware so called from resemblance of its lustrous transparency to the shiny surface of the shells. The shell's name in Italian is from porcella "young sow," fem. of Latin porcellus "young pig," diminutive of porculus "piglet," diminutive of porcus "pig" (see pork (n.)). According to an old theory, the connection of the shell and the pig is a perceived resemblance of the shell opening to the exposed outer genitalia of pigs.
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]