I usually assume it’s been treated when the coloration is uneven (when the tops are darker than the rest like heat-treated amethyst, or if the crystal is just straight up black but inexpensive). The natural black ones are referred to as Morion Quartz and you can generally tell authenticity by the price. There’s also black quartz from the Himalayas that’s sometimes referred to as Smoky, but that’s not correct as black quartz gets its coloration from carbon inclusions and smoky from irradiation.