With a density of 6.5-7.0 (quartz is 2.65, calcite is 2.71), wulfenite is among the densest of all transparent minerals. However, because most of the elemental bonding in wulfenite is due to weak ionic bonds, wulfenite is soft, registering just 2.5-3.0 on the Mohs scale, or slightly softer than calcite.