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A song lyric from a modern western pop-band isn't the sort of place I would look for authoritative information about cultural practices in pre-communist China.
There are accounts of such practices in Chinese death rituals (in Singapore)
Historically, the majority of the Chinese population have been extremely poor rural peasants. I'd be surprised if pearls or mercury were within the means of any but a tiny wealthy elite.
Upvote:1
Apparently the Wikipedia article is likely wrong. http://ghanisquest.livejournal.com/82903.html
She delivers a few lines before a large, black pearl is inserted into her mouth, signifying her death. Before dying, however, she tells the boy that men are not permitted in the palace past dusk. link
and
Ancient Chinese believed that pearls were conceived in the brains of dragons. In imperial china, the black pearl was regarded as a symbol of wisdom. As such, it was guarded between the teeth of the dragon. link
Upvote:2
Śarīra is a tradition / belief referring to Buddhist relics, although in common usage it usually refers to pearl or crystal-like bead-shaped objects that are purportedly found among the cremated ashes of Buddhist spiritual masters.