Your thesaurus and your treasure are the same word — both mean "storehouse," one for gold, the other for words.
A quantity of precious metals, gems, or other valuable objects. Also used figuratively for anything highly valued or cherished.
From Old French tresor, from Latin thesaurus meaning 'storehouse, treasure,' borrowed from Greek thēsauros ('store, treasure, treasury'), possibly from a pre-Greek source or from the root of tithenai ('to put, place'). Key roots: *dheh₁- (Proto-Indo-European: "to put, place, set"), thēsauros (Greek: "storehouse, treasure"), thesaurus (Latin: "treasury, collection").
The word "thesaurus" — that reference book of synonyms sitting on your shelf — is the exact same word as "treasure," just taken directly from Latin rather than through French. Peter Mark Roget published his famous Thesaurus in 1852, using the word in its original sense of "storehouse" — a storehouse of words. Meanwhile, the ancient Greeks built literal thesauroi at religious sanctuaries