Named stone

Gunflint silica

Also called GFS, Gunflint chert, Animikie silicate (in part)

Gunflint silicaChert (and flint)Midwest/Ohio Valley

Description

Gunflint silica is a chert from the same Precambrian Gunflint Iron-Formation as jasper taconite, exposed between the east end of Gunflint Lake and North Lake on the Minnesota-Ontario border. Here the clear chalcedony matrix dominates, with scattered black specks like pepper. Pieces range from nearly colourless or grey-blue to black, and some have red streaks. A broken surface looks like a broken ice cube. Good pieces break cleanly, but most of the stone splits along small joints into blocks, so large blanks were hard to get. It turns up in Paleoindian sites but was used more by later people, whose smaller tools suited it. No prehistoric quarry has been found yet. Artifacts reach Mille Lacs Lake, Granite Falls and Pine City.

How to identify it

Colour: Bakken: translucent grey to bluish grey, sometimes nearly colourless. Lindenberg: transparent to black (white, light grey, tan, dark grey, brownish grey, bluish black), most often black because of magnetite; some pieces carry enough hematite for red streaks.
Texture: Fracture surface like a broken ice cube (Bakken). Either granular and mottled with relict ooliths, or more even chert with fine parallel iron-oxide lenses (Lindenberg). High-grade pieces break conchoidally; most lower-grade stone breaks into blocks (Harrison et al. 1995, via Lindenberg). Working quality below jasper taconite (Bakken).
Luster: Waxy to glossy on fresh surfaces (Harrison et al. 1995, via Lindenberg).
Inclusions: Scattered small black granules like pepper grains (Bakken); magnetite, hematite and organic matter (Lindenberg).
Heat treatment: Romano's experiments improved it, most of all the lower-grade stone (via Lindenberg).

Geology

A chert of the Middle Precambrian Gunflint Iron-Formation (Animikie Group), formed when iron minerals precipitated in shallow seas and silica then cemented the sediment (Lindenberg). Samples are also reported from deep iron mines in Minnesota, and it occurs in glacial drift (Romano, via Bakken).

Where it comes from

Midwest/Ohio Valley

States

Minnesota

Quarries and outcrops

Gunflint Lake to North Lake, Minnesota-Ontario border (Lindenberg; Romano via Bakken)
Secondary cobbles from Gunflint Lake to the Pigeon River and in northeastern Minnesota drift (Lindenberg)

Used by

Paleoindian (uncommon: blocky fracture limited blank size) and, more often, later prehistoric periods (Harrison et al. 1995, via Lindenberg)
Artifacts at Malmo on Mille Lacs Lake, Granite Falls, Island Lake (Reservoir Lakes) and Pine City; debris near Morton on the Minnesota River (Lindenberg)
Present at many Lake Superior North Shore sites in Minnesota, usually with jasper taconite (Mulholland et al. 2011)

Look-alikes

Stones that are easy to confuse with Gunflint silica.

Biwabik silica (Mesabi Range): the closest match by eye and in thin section; Romano finds it duller and more matte (Lindenberg)
Dark jasper taconite and thinly banded Kakabeka chert (Lindenberg)

Sources

Lindenberg, Sheila Marie. 1996. Source Characterization of Western Lake Superior Region Lithics Using Neutron Activation Analysis: A Geoarchaeological Study. M.S. thesis, University of Minnesota.conservancy.umn.edu (opens in a new tab)
Bakken, Kent E. (written about 1995; PDF issued later). Chipped Stone Raw Materials. Chapter 6 in Stone Tools of Minnesota. Minnesota Office of the State Archaeologist, St. Paul.mn.gov (opens in a new tab)
Mulholland, Susan C., Stephen L. Mulholland, Jennifer R. Hamilton, and Stacey Stark. 2011. Points and Pits: Archaeological Investigations in Minnesota's Region 9, the Lake Superior Shore, Carlton, Cook, Lake, and St. Louis Counties, Minnesota. Duluth Archaeology Center Report 11-22.mn.gov (opens in a new tab)

All materials