We don't have synonyms for brillant yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are brilliant, brisant and brillance.
"1891: "And the ring?" I asked, glancing at a remarkable brilliant which sparkled upon his finger. -- Arthur Conan Doyle, 'A Case Of Identity' (Norton 2005 p.76)"
"1891: "And the ring?" I asked, glancing at a remarkable brilliant which sparkled upon his finger. -- Arthur Conan Doyle, 'A Case Of Identity' (Norton 2005 p.76)"