Brilliant may have crossed more language families than almost any word in English. The leading theory traces it from French brillant ('shining') to Italian brillare ('to sparkle') to Latin beryllus ('beryl gemstone') to Greek bḗryllos, to Prakrit veḷuriya, to a Dravidian root meaning 'to be white' or 'to shine'. From southern India to every English dictionary.
The beryl connection is debated but influential. Beryl is a pale green mineral that, when cut, produces the gemstone emerald. Its sparkle may have given Italian the verb brillare in the 16th century, though some scholars prefer a simpler onomatopoeic origin.
English borrowed brilliant from French in the 1680s, initially for physical light. The intellectual sense — a brilliant scholar, a brilliant idea — appeared by the mid-18th century. The metaphor of intelligence as light runs deep in European languages: illumination, enlightenment, bright student, dim-witted.
In British English, brilliant acquired a colloquial sense of general approval ('That's brilliant!') by the 20th century. The word that possibly began as a description of South Indian gemstone lustre now serves as casual praise in Bristol pubs.