Today it means: A rare monoclinic-prismatic lead copper chromate arsenate hydroxide mineral, which is generally green to yellow and translucent to transparent, and has a Mohs hardness of 2.3, a specific gravity of 6.27, and a Strunz classification of 7.FC.10. Full definition →
From Latin fornāx, fornāc- (“furnace”) -ite, named in 1915 after Lucien Lewis Forneau (whose surname resembles French fourneau (“stove”), which, like fornāx, is ultimately from Latin furnus (“oven”)).
Etymology from Wiktionary, CC BY-SA 4.0.