Lake Superior agate

Lake Superior agate
Lake Superior agate from Duluth, Minnesota
General
Category	Tectosilicate minerals, quartz group, chalcedony variety, agate variety
Formula	SiO2 (silicon dioxide)
Crystal system	Trigonal (quartz) or monoclinic (moganite)
Identification
Color	Red, orange, pale yellow, brown, white, grey, rarely blue
Crystal habit	Cryptocrystalline silica
Cleavage	None
Fracture	Conchoidal, with very sharp edges
Mohs scale hardness	6.5–7
Luster	Waxy, vitreous when polished
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	2.58–2.64
Density	2.6 g/cm³
Refractive index	1.530–1.540
Birefringence	Up to +0.004 (B-G)
Pleochroism	Absent
Common impurities	Iron oxides (goethite, hematite, limonite)[1]

Lake Superior agate is a type of agate found primarily near the shores of Lake Superior. It can also be found in interior regions of the U.S. states of Minnesota, Michigan, Wisconsin, Iowa, Nebraska, Kansas, and Missouri, and in the Canadian province of Ontario. As a gemstone, it is valued by collectors for its vibrant, iron-colored bands in shades of red and orange, but they may also be white, grey, pale yellow, or brown. Believed to be the world's oldest agates,^[2] Lake Superior agates formed within the basaltic lava flows left behind from the Midcontinent Rift about 1.1 billion years ago. They are not named after the lake, but rather the Lake Superior Till, a Pleistocene glacial deposit in which they can be found.^[3] In 1969, the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.^[4]