EMERALD CRYSTAL POCKETS OF THE HIDDENITE DISTRICT, ALEXANDER COUNTY, NORTH CAROLINA

The Hiddenite District has produced North America's largest emeralds since the early 1880s. The largest is a 1,869 carat crystal found in 2003 on the North American Emerald Mines property located on the NE side of the 2 x 5 km district. 70% of the district's estimated 50,000 carats of emerald production have come from this property and includes 15 of North America's 20 largest emeralds. Recent mine exposures provide new insights to the geology and mineralogy of these unique deposits.

Emeralds are hosted by complex Alpine Type quartz veins cutting Silurian(?) metasedimentary rocks of the tectono-stratigraphic Piedmont terrane. This 160 x 1,100 km crustal block has a long deformational history involving multiple continental collisions. Host rocks are a sequence of fine grained siliceous sediments that reached sillimanite grade metamorphism and are mapped as migmatitic biotite gneiss of the Brindle Creek thrust sheet.

The veins originated as hydrothermal filling of ductile/brittle sites during the waning stages of Alleghanian deformation about 250-200 Ma. Typical veins exhibit shattered cryptocrystalline quartz (crackle breccia) overlying pockets partially filled with breakdown crystal breccia. Pockets occur in about 50% of the veins and emeralds occur in about 50% of the pockets. Breakdown crystal breccia is a collapse deposit of crystals that have fallen from the ceiling and walls of the pockets. Re-growth, over-growth and cementation of fallen crystals are common. Primary pocket minerals include quartz, muscovite, albite, calcite-siderite-dolomite, rutile, clay, beryl, hiddenite, sulfides, and graphite. Emeralds are found as individual collapse fragments and as free-standing crystals still attached to pocket walls. Narrow wall-rock alteration halos are common peripheral to veins and crystal pockets.

Emerald values vary widely with collector specimens up to $1,200/ct and cut gemstones up to $79,000/ct. Remarkably, emerald crystals average over 50 carats each and crystals over 1,000 carats account for 8% of the total production. Pockets containing up to 3,500 carats of emerald are documented.