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Polka Dot Agate
A beautiful translucent blue to white agate with suspended dots of various colors.
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
Polka dot agate: Several cabochons of polka dot agate cut using material from the Priday Agate Beds of central Oregon. These cabs show only part of the color diversity of the dots and the various types of background material. The cab in the center of the bottom row measures 36 x 25 millimeters in size.
What is Polka Dot Agate?
Polka dot agate is an orbicular agate produced from the Priday Agate Beds of central Oregon. It has an appearance that sounds just like its name. It is a semitransparent to semitranslucent agate with suspended round "dots" of contrasting colors.
It is a favorite Oregon gemstone.
Polka dot's base colors range from blue to white to pale yellow. The dots can be almost any combination of yellow, orange, red, brown, or black. Nature has also marked some specimens with black dendrites. Other specimens are cross-cut by intrusions of brown jasper. The diaphaneity ranges from almost transparent to almost opaque. This color and geometry makes some of the most interesting and beautiful types of agate that you will ever see.
The dots in the agate range in size from nearly invisible up to about 1/4 inch across. Most of them are completely agatized, but some appear to be a soft iron oxide material similar to limonite or hematite. Most of the dots are sharp, concentric spheres. Others are spherical but with a slightly irregular outline.
Close-up of polka dot agate: A close-up of a sawn surface of polka dot agate showing orange and brown dots at the surface and below the surface of the translucent agate. The larger dots are about four millimeters across.
Native Americans Were the First Miners
The first people to mine polka dot agate were Native Americans. They valued the agate because it could be knapped into sharp tools such as scrapers, blades, and projectile points. They carried, worked, used, and traded these materials across what is now the northwestern United States and southwestern Canada.
Today, West Coast Mining company operates the polka dot agate mine, located near the community of Madras in Jefferson County, Oregon. They produce rough, slabs, cabs, and other items made from polka dot agate. They also own mines that produce Opal Butte opal, Owyhee picture jasper, Paiute agate, amethyst sage agate, and Rock Butte picture jasper.
Some of these mines are open to the public for fee mining. On a limited schedule, they are open to people who want to visit the mine, pay a fee, look for agate, and keep what they find. The company also sells agate, jasper, and opal directly to the public and online.
Polka dot agate rough: Polka dot agate rough purchased from West Coast Mining's eBay store. The top piece is about eight centimeters across and has a background color that ranges between white and slightly blue. The bottom piece has a few black dendritic markings along a vertical path in the center of the specimen.
Polka dot agate is a favorite lapidary material of many people. It cuts nicely on a diamond or carbide wheel and produces a brilliant luster when polished on felt with aluminum, tin, or cerium oxide. It can be used to make beautiful tumbled stones in a rock tumbler.
The colorful dots make interesting cabs regardless of their density. Marking up a slab into cab outlines can be a pleasurable challenge. Most people really enjoy specimens of polka dot agate with a blue base color and have given it a cool nickname -- "blue ice."
Polka dot agate from Africa: Cabochons sold as "polka dot agate," cut by an Indian vendor who reports that they were cut from material mined in Africa.
In the United States, the Priday Agate Beds are the popular source for polka dot agate. Similar-looking material is also found in other parts of the world. The two oval cabochons in the accompanying image were made from "polka dot agate" that was mined in Africa and looks a lot like material from the Priday Agate Beds. People who have cut a lot of Priday polka dot can probably tell the difference, but inexperienced people might not.
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