polydymite

from The Century Dictionary.

• noun A sulphid of nickel, occurring in isometric octahedrons and in massive forms, of a light-gray color and brilliant metallic luster. A ferriferous variety from Ontario carries a small amount of platinum.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy An isometric supergene sulfide mineral, dark violet gray to copper-red, associated with the weathering of primary pentlandite nickel sulfide.