The Lake Superior Agate

Minnesota Gem: The Lake Superior Agate Its wide distribution and iron-rich bands of color reflect the state gemstone's geologic history in Minnesota Scott F. Wolter N 1969 the dream of Mrs. Jean it was the best choice. The agate IDahlberg was realized: The Lake reflects many aspects of Minnesota. Superior agate was designated by the It was formed during violent, fiery lava Minnesota Legislature as the official eruptions that occurred in our state state gemstone. about a billion years ago. The stone's The late Mrs. Dahlberg, long-time predominant red color comes from rock hound and ardent fan of the agate, iron, the major industrial mineral in testified before the state legislative our state. Finally, the widely distrib- committee considering the bill. She uted agate reveals the impact of gla- knew how perfect the Lake Superior cial movement across Minnesota agate was for the state gemstone. 10,000 to 15,000 years ago. However, there were other logical The Lake Superior agate is inti- candidates. The brazen red Bing- mately related to two prominent geo- hamite and blazing yellow silkstone, logic features of Minnesota: the both iron-rich jaspers found in the majestic and rugged lava flows Cuyuna iron range area, were logical exposed along the shoreline of Lake selections. Thompsonite, the beau- Superior and the lake-freckled glacial tiful and popular zeolite mineral found debris that covers nearly all the state. only in Minnesota on an isolated More than a billion years ago, the stretch along Lake Superior, was North American continent began to another strong candidate. Pipestone, split apart into two separate conti- crafted into peace pipes by Indians in nents. This catastrophic event, Pipestone, was another possibility. spurred by molten rock moving deep But a closer look at the history of within the earth, poured out thou- the Lake Superior agate proves that sands of lava flows. These flows now Top, left: Top row shows original stones from which agates in bottom row were 37 cut and polished. Bottom, left: Bands of red, white, and blue appear in this 2.5-pound agate found near Cloquet in the early 1950s. Minnesota Gemstone are exposed along the north and south shores of Lake Superior. Quartz-rich. If this drastic event had not stopped shortly after it began, what is now Minnesota might have comprised parts of two separate con- tinents. When the continental part- ing did stop, the remaining thick, heavy lava pile pressed down on the earth's crust producing a long trough — the Superior trough, which eventually became the Lake Superior basin. These lava flows also became the birthplace of Lake Superior agates. Water vapor and carbon dioxide became trapped within the solidified flows in the form of millions of bub- bles or pockets. Later, groundwater carrying ferric iron, quartz, and other dissolved minerals passed through the trapped gas pockets or vesicles. These quartz-rich groundwater solutions Beaver Bay Agate Shop owner James crystallized into concentric bands of Haase displays North Shore agates. fine-grained quartz called chalcedony. Over the next billion years, a few the beginning of the Great Ice Age. of these quartz-filled, banded vesi- A vast sheet of ice formed over what cles — agates — were freed by run- is now Hudson Bay in Canada and ning water and chemical expanded. Eventually it sent lobes of disintegration of the lavas. The vast mile-thick ice into Minnesota and majority, however, remained lodged other parts of North America. in the lava flows until the next major One particular lobe of ice, the geologic event that changed them and Superior lobe, moved into Minne- Minnesota. sota 10,000 to 15,000 years ago. It fol- About 2 millon years ago, the lowed the agate-filled Superior world's climate grew colder signaling trough. The glacier picked up the surface agates and carried them south. Scott F. Wolter, a geologist at Twin City Its crushing action and cycle of freez- Testing, St. Paul, is the author of The Lake ing and thawing at its base freed many Superior Agate, reviewed in the Novem- agates from within the lava flows and ber-December 1987 issue of the Volunteer. transported them, too. The advanc- 38 THE MINNESOTA VOLUNTEER rial called "moraines.'' Cutting into these moraines were rivers and streams swollen with water from melting glacial ice. The streams carried sand and gravel, including agates, and deposited these sediments as "outwash." These streams performed the final abrasion and rough polishing of the agates. The Lake Superior agate differs from other agates found around the world in its rich, red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. Iron from Minnesota's iron range pro- vided the pigment that gives the gemstone its beautiful array of color. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands. The gemstone comes in various Top: Octagonal patterned 2.5-pound agate. sizes. The gas pockets in which the Bottom: Two-pound red and white agate. agates formed were primarily small, about the size of a pea. A few Lake ing glacier acted like an enormous rock Superior agates weigh more than 20 tumbler, abrading, fracturing, and pounds, about the size of a bowling rough-polishing the agates. ball. Such giant agates are extremely rare, but no doubt others lie waiting Eye-catching Patterns. The Supe- to be discovered. rior lobe spread agates and other The most common type of Lake debris throughout northeastern and Superior agate is the fortification agate central Minnesota and extreme with its eye-catching banding pat- northwestern Wisconsin. A period of terns. Each band, when traced around warmth triggered melting and retreat an exposed pattern or "face," con- of the lobe, leaving the large body of nects with itself like the walls of a fort, freshwater now called Lake Superior. hence the name fortification agate. Abrasion of the agates, however, was A common subtype of the fortifi- not yet complete. cation agate is the parallel-banded, Along the margins of the glacier onyx-fortification or water-level agate. were ridges of ice-transported mate- Perfectly straight, parallel bands occur JANUARY-FEBRUARY 1988 39 Minnesota Gemstone Left: Agate hunter on Lake Superior beach. Above: Gray area or. map shows approxi- mate distribution of agate in Minnesota — Little Marais in northeast, Walk- er in north-central, Will- mar in west-central, Al- bert Lea in south. over all or part of these stones. The wave-battered rocky beach. We call straight bands were produced by these, appropriately enough "water- puddles of quartz-rich solutions that washed" agates. crystallized inside the gas pocket My favorite Lake Superior agate has under very low fluid pressure. The only two colors, fire-engine red and parallel nature of the bands also indi- snow-white alternating bands. I call cates the agate's position inside the these agates "candystripers." lava flow. Finally, the rarest Lake Superior agate is the one that recurs in a col- All-timers. Probably the most lector's dreams but is discovered in popular Lake Superior agate is also reality perhaps once in a lifetime. On one of the rarest. The highly trea- average only one out of every 10,000 sured eye agate has perfectly round agates fits this description. They are bands or "eyes" dotting the surface of the ones weighing 2 pounds or more the stone. and having perfect shape, color, and Occasionally, collectors find a banding quality. They are the ones gemstone with an almost perfectly called "all-timers." smooth natural surface. These rare A few all-timers have been col- agates are believed to have spent a lected and displayed at rock and gem long time tumbling back and forth in shows. The possibility of finding one the waves along some long-vanished, keeps agate hunters searching. 40 THE MINNESOTA VOLUNTEER Clues to Finding the State Gem You have decided to hunt for Lake Superior mon are shades of rust-red and yellow. agates, but how do you know what to look • Translucence is an optical feature pro- for? duced by chalcedony quartz, the principal There is no simple answer. Usually, the constituent of agates. The quartz allows light richly colored banding pattern is not well to penetrate, producing a glow. Sunny days exposed and prospectors must look for other are best for observing translucence. clues to the presence of agates. • A glossy, waxy appearance, espe- The following characteristics will help you cially on a chipped or broken surface, is identify agates in the field. another clue. • Band planes along which the agate has • A pitted texture often covers the rock broken are sometimes visible, giving the rock surface. The pits are the result of knobs or a peeled texture. It looks as if the bands were projections from an initial layer of softer min- partly peeled off, like a banana skin. eral matter deposited on the wall of the cav- • Iron-oxide staining is found on nearly all ity in which the agate formed. Later, when agates to some degree, and generally cov- the quartz that formed the agate was ers much of the rock. Such staining can be deposited in the cavity, these projections left many different colors, but the most com- impressions on the exterior. Of course, the word "gemstone" often are cut with diamond saws into implies that a stone can be used as a thin slabs, which then are cut into jewel when cut and polished. The various shapes. One side of the shaped Lake Superior agate certainly quali- slab is polished producing fine jew- fies, although only a fraction of the elry pieces and collectible gems called stones are of the quality needed for cabochons.

Load more