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Agate, photo 1
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Rock type

Agate

Also called Lake Superior agate, Moss agate, Banded agate

Agateagate (banded or patterned chalcedony)All regions

2 photos

Description

Agate is banded chalcedony, usually showing concentric bands of different colours. Moss agate lacks bands and holds tree-like dark inclusions of manganese and iron oxides. Agates form where silica-rich water fills gas pockets or cavities in rock. Lake Superior agates formed about a billion years ago in Minnesota lava flows, were freed by weathering, and were spread by glaciers, so they turn up in gravels across Minnesota; their red comes from iron. Agates are mostly collected as gems, but the USGS says early peoples worked agate and other chalcedony varieties into tools. Tallahatta agate in Mississippi is an opal-rich chalcedony that fills joints and burrows, translucent yellow to orange.

How to identify it

Colour
Banded in several colours; Lake Superior agate is mostly red from iron; Tallahatta agate is translucent yellow to orange; Montana moss agate is grayish-white to gray with black, brown or red dendrites.
Texture
Fine, fibrous chalcedony; fortification agate shows bands like fort walls.
Luster
Waxy.
Inclusions
Moss agate: dendritic growths of manganese and iron oxides. Banded agate: concentric bands.
Cortex
A pitted surface is common on Lake Superior agate, caused by the first layer of softer mineral in the cavity.

Geology

Agate forms as chalcedony fills voids. In Minnesota, Lake Superior agates formed about a billion years ago when gas pockets in lava flows were filled by iron-bearing, quartz-rich water that crystallised as banded chalcedony; erosion later freed some, and glaciers 10,000 to 15,000 years ago spread them widely. Tallahatta agate occurs in the Basic City Member of the Tallahatta Formation in Mississippi and Alabama as mineral fillings of joint fractures, fossil burrow casts and mollusk moulds beside Tallahatta quartzite, and its laminar bedding can spoil knapping.

Where it comes from

All regions

States

Quarries and outcrops

  • Lake Superior region, Minnesota (the state gem)
  • Tallahatta Formation, east-central Mississippi and west-central Alabama
  • Montana (moss agate cobbles)

Used by

  • Agate, along with other chalcedony varieties, was used by early peoples for tools and weapons (USGS)

Look-alikes

Stones that are easy to confuse with Agate.

  • Chalcedony without banding
  • Jasper (opaque) and flint
  • Agatized coral of Florida (Tallahatta agate is similar but with laminar bedding)

Sources

  1. U.S. Geological Survey. 1995. Gemstones: Chalcedony (SP-14-95), Mineral Information archive.apps.usgs.gov (opens in a new tab)
  2. Minnesota Department of Natural Resources. Minnesota's State Gem: The Lake Superior Agate (Digging into Minnesota Minerals).dnr.state.mn.us (opens in a new tab)
  3. Mississippi Department of Environmental Quality, Office of Geology. Lithic Materials (web page; updated December 2023).mdeq.ms.gov (opens in a new tab)
  4. 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)

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