Brazilian amethyst turns light yellow and dark yellow to red brown when heated. Some smoky quartzes turn yellow earlier. All heat-treated stones have a red tint. The natural citrines are mostly pale yellow. Treated citrines show no pleochroism. The trade often calls citrines “topazes”. This is incorrect, even when qualified as Bahia-topaz, gold-topaz, Madeira-topaz etc
Natural citrine is rare. Deposits are in Brazil, Malagasy Republic, the U.S., Spain,Russia,France and Scotland. Well-colored, transparent specimens are used as ringstones and pendants. Less attractive stones are made into necklaces or ornaments.

*Info by Walter Schumann “Gemstones of the World”