Polkadot agate is a trade variety of agate distinguished by small, rounded, contrasting spots distributed across a translucent or banded chalcedony host. The spots, which range from a millimetre to several millimetres in diameter, are produced by the rhythmic precipitation of mineral impurities during the original silica deposition and may appear white, red, brown, or black against a paler ground. The variety is cut for cabochons, beads, and ornamental objects and trades in the broad category of decorative agates rather than as a recognised gem species in its own right.

Composition and formation

Like all agate, polkadot agate is cryptocrystalline silica — chalcedony — formed from silica-bearing groundwater that infiltrates cavities in volcanic and sedimentary host rocks and precipitates in successive layers as the chemistry of the solution shifts over time. The polkadot pattern arises when the precipitation includes localised concentrations of iron, manganese, or other oxide impurities that nucleate as small spheres or rounded patches rather than as continuous bands. Where the host chalcedony is translucent, the spots stand out as dark inclusions; where the host is banded, the spots may overlay or interrupt the agate banding.

Properties match those of agate in general: hardness 6.5 to 7 on the Mohs scale, refractive index near 1.535, specific gravity around 2.6, and a waxy to vitreous lustre on a polished surface. The material accepts a high polish on cerium oxide and is straightforward to work with conventional cabbing equipment.

Sources

Polkadot agate occurs in agate-producing regions worldwide, with documented production from Brazil, India, the United States, and Mexico. The variety is not associated with a single celebrated locality in the way that Lake Superior agate or Botswana agate are; it is rather a pattern type that appears in agate fields globally and is selected at the rough stage by cutters looking for the distinctive spotted figure.

In the trade

Polkadot agate is sold primarily for ornamental and lapidary use rather than for fine jewellery. Cabochons are cut to display the spots prominently, often by orienting the slab so that a strong contrast between dot and ground is presented at the dome. Beads and tumbled pieces are common in the broader semi-precious market. Pricing is in the range of decorative agates and reflects pattern quality and slab thickness rather than rarity.

Identification

Identification follows standard agate criteria: cryptocrystalline silica, refractive index, specific gravity, and the visible chalcedony fibre under crossed polarisers. The polkadot pattern itself is descriptive rather than diagnostic and is not used in formal mineralogical classification. Care should be taken to distinguish polkadot agate from dyed material with painted or dyed spot patterns — natural spots are integrated into the silica matrix and survive sectioning, while applied colour is typically superficial and inconsistent across cut faces.

Further reading

• GIA — Agate reference
• International Gem Society — chalcedony and agate varieties
• GIA Gem Encyclopedia — quartz