From Latin efflōrēscere ('to begin to bloom out'), from ex- + flōs ('flower'), from PIE *bʰleh₃- ('to bloom'). Entered English with both its botanical sense (flowering, peak blooming) and its chemical sense (salts 'flowering' into powder on surfaces) — alchemists' poetic metaphor made permanent.
The process or state of flowering; a period of peak development or productivity; in chemistry, the formation of a powdery crust on a surface through loss of water of crystallization.
From French 'efflorescence', from Latin 'efflōrēscentia', from the inchoative verb 'efflōrēscere' ('to begin to bloom'), composed of 'ex-' ('out') + 'flōrēscere' ('to begin to flower'), itself the inchoative of 'flōrēre' ('to flower, bloom'), from 'flōs' (genitive 'flōris', 'flower'). Latin 'flōs' derives from PIE *bʰleh₃- ('to bloom, flower'), which also produced Old English 'blōwan' ('to bloom'), English 'blow' (in the archaic sense 'to blossom'), 'bloom', 'blossom', 'blade' (originally a leaf), and 'blood' (possibly via the sense of something that springs forth). The word entered English simultaneously in its literal botanical sense and its chemical sense — alchemists noticed that certain salts
The chemical sense of 'efflorescence' — a white powdery crust forming on brick, concrete, or stone — is one of the most literal metaphors in science. Alchemists saw dissolved salts migrating to a surface and crystallizing into delicate white patterns and thought it looked like flowers blooming on stone. Builders today still use this 17th-century poetic metaphor when diagnosing the white stains