Board Thread:Off Topic/@comment-27144409-20170705210718/@comment-25936766-20170815224345

GreyStark wrote:

Brilliant is apparently Russian for diamond Not sure about Russian, but it's at least English. But I think it's more of an informal use.

According to what I found on the internet, "Brilliant" = "finely-cut gems, especially diamonds, cut in a particular form with numerous facets so as to maximize light return through the top (called "table") of the stone."

If Arthur Conan Doyle's use of it once is any indication, that meaning has been in use since before the XXth century.