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
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.
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 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."
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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