A Lake Superior Agate (LSA) is a distinctive type of banded chalcedony, a microcrystalline variety of quartz, found predominantly within the glacial deposits of the Lake Superior basin. These stones originated within the ancient basaltic lava flows of the Midcontinent Rift System approximately 1.1 billion years ago. Groundwater rich in silica and iron permeated the gas pockets left in the solidified lava, slowly depositing the layers that form the agate. The resulting nodules were scattered and polished by glacial movements, making them a prized find throughout the Upper Midwest.
The Defining Color Palette
The most distinguishing characteristic of an LSA is its vibrant, warm color range, which sets it apart from many other agates globally. The dominant colors include deep reds, fiery oranges, sunny yellows, and rich browns, often arranged in alternating bands with white or gray layers. These striking hues are caused by the inclusion of iron oxide, specifically hematite, which leached into the stone’s structure during its formation. The concentration and oxidation of the iron determine the depth and intensity of the color, typically yielding the saturated rust-reds for which these agates are most famous.
Internal Structure and Banding
The internal structure of an LSA reveals the layered history of its formation, displaying specific patterns fundamental to its identification. The most common pattern is fortification banding, characterized by angular, zigzag, or citadel-like lines that resemble the walls of a fort. These complex, geometrical patterns occur because each band, when traced around an exposed surface, connects back to itself. Another element is the parallel-banded or water-level agate, which features straight, horizontal bands, indicating the silica solution settled horizontally due to low fluid pressure. Collectors also seek eye formations—perfectly round, concentric circles that appear like bullseyes dotting the stone’s surface.
External Characteristics and Common Forms
In its natural, rough state, an LSA often has a deceptively dull exterior that can make it difficult to spot among ordinary rocks. The outer layer, known as the rind, is frequently waxy, pitted, or pockmarked, sometimes resembling common limestone. This pitted texture is an impression left by the host rock, a remnant of the gas bubble’s interior where the agate originally formed. LSAs are typically found as irregular spheres or nodules because they formed in the rounded gas pockets of ancient lava flows, ranging in size from pea-sized to over twenty pounds.
Differentiating Features
Identifying a genuine Lake Superior Agate requires combining visual cues to distinguish it from common look-alikes such as jasper or plain quartz. The most reliable identifier is the unique combination of the warm iron-oxide colors and the specific patterns of fortification banding. A practical technique for confirmation is backlighting, which involves holding the stone up to a strong light source. Since the agate is composed of translucent chalcedony, light passes through the edges or thinner sections, giving the stone a subtle, internal glow and immediately ruling out opaque imitators.