So the eyebrow-raising Korean birth customs didn't start with gold-plating umbilical cords or turning them into personal seals.
Korean royalty used to [still does? I don't know] place a prince or princess's placenta in a placenta jar, then bury it, thereby assuring the kid a long, healthy life. Umbilical cord, too. That's all fine and good. But it went beyond that.
Here's part of an explanation from the Korean Museum of taebang, the ritual of caring for the tae, aka the placenta and cord, also known as the "birth relics":
A traditional Korean belief holds that the placenta and umbilical cord to contain the vital energy of a new born. Some people even used to think that they had the power to determine the future of a human being.As if he didn't do enough by creating the Korean alphabet, King Sejong the Great [1397-1450] also established the largest collection of taesil in Korea. The placentas and umbilical cords of all 18 of his sons and his grandson were enshrined at the foot of Seonseoksan Hill.
These relics of the fetal life were therefore carefully preserved in jars, and placed in a special building dedicated to them [called taesil.]
Hosting a royal taesil was a great honor for a town.
Places chosen for a royal taesil frequently received a new, higher administrative status,
or exemptions on taxes and services due to the state.
The placenta jar above is the only picture I could find of both the inner and outer jars. It's in the blue glaze Buncheong style, and is decorated with a stamped chrysanthemum pattern. It is in the collection of the Korean University Museum.
Buncheong Placenta Jar [ocp.go.kr]
The place where buried the placenta, Taesil (Placenta chamber) [korea-museum.go.kr]
Taejang Ceremony [wikipedia]
Sejong the Great Princes' Taesil google maps via virtualglobetrotting.com]