The answer is color.
Among the sulfides are included the minerals having a general type formula MmSp, where M is a metal (Ag, Cu, Pb, Zn, Fe, Ni, Hg, As, Sb, Mo, Hg, Tl, V). some of these sulfides have a characteristic yellow color such as:
* Arsenic sulfide, orpiment is a mineral easily recognizable by its golden yellow to orange yellow color, its very easy cleavage in transparent and flexible plates, recalling that of the micas, and by its low hardness, allowing to scratch it with the nail.
* Iron sulfide, pyrite is a very common mineral and present in many types of rock. Its determination is relatively easy because of its brass yellow color, its metallic luster and the geometry of its frequent crystals derived from the cube.