Rutile

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A sharp lustrous well-formed crystal of rutile, this is the variety referred to as struverite. In very good condition.Struverite is a black, tantalum-rich variety of rutile that typically forms in granitic pegmatites. Geologically, it occurs when niobium and tantalum ions substitute for titanium within the rutile crystal lattice, often as a result of late-stage magmatic differentiation. The mineral is characterized by its high specific gravity, metallic to sub-metallic luster, and its typical occurrence as compact, iron-black crystals or granular masses. It was historically named in honor of the Italian mineralogist Giovanni Struver and is often found in association with other rare-element minerals like columbite, tantalite, and beryl. While rutile is common, the struverite variety is a significant indicator of highly evolved pegmatite systems and has been historically documented in classic localities such as those in Northern Italy and the Black Hills of South Dakota. Because of its dark color and crystal habit, it can be difficult to distinguish from other heavy black oxides without detailed chemical analysis or specific gravity testing.