Sphalerite, also known as Blende or Zinc Blende, is the major ore of zinc. When pure (with little or no iron) it forms clear crystals with colours ranging from pale yellow (known as Cleiophane) to orange and red shades (known as Ruby Blende), but as iron content increases it forms dark, opaque metallic crystals (known as Marmatite). Sphalerite's luster really sparkles. It's unusually high index of refraction (2.37 - 2.42) produces a fire greater than diamond's. Because sphalerite is a rather soft gemstone (3.5 - 4 on the mohs scale) it is generally used as a collectors gem.