Amethyst
Amethyst fades in sunlight. Heat it and it turns yellow — most commercial citrine is heat-treated amethyst.
Traditionally associated with clear thinking, restraint and calm; the classic stone of sobriety.
Care. Warm soapy water only. Keep out of direct sunlight, which fades the colour permanently.
Spotting a fake. The convincing fakes are synthetic amethyst, chemically identical and detectable only by lab. Glass is easier: look for bubbles and unnaturally even colour. Real amethyst usually shows colour zoning.