A small ornamental box or chest for holding jewels, letters, or other valuable objects.
The Goblet of Fire was kept in a jewel-encrusted casket.
From the Latin word 'capsa', related to capere 'to hold'.
In North America, the term 'casket' is synonymous with 'coffin,' which has confused some readers. Anti-Harry Potter folks circa 2000 used this misreading of the text as one 'proof' that the books were 'inappropriate for children' and therefore ought to be banned from the school library where Steve Vanderark was the librarian.