Late 17th-century London: Isaac Newton keeps vigil on St Martin’s Street, bent over crucibles where gold and mercury confront each other. On his desk, shards of amethyst act as a counter-test, their mineral violet betraying the slightest impurity in molten metal. In nearby salons, people whisper about a perfect tincture, one that reveals without dazzling. Amethyst Regent holds on to that union of science and liturgy: light kept in check, authority spoken softly. A study-chamber violet for those who decide without raising their voice.