Kentucky Agate: State Rock and Mineral Treasure of the Commonwealth

Chapter 1. Types of Agates

5 1 Types of Agates An agate is a concretion or nodule of quartz, of the variety chalcedony, which is silicon dioxide. These glasslike round or spherical nodules contain banded, variegated, or mottled coloring achieved by various minerals, and the banding is often reminiscent of the concentric rings of a tree. Agates can be solid or have a geodal center. Agates can range from as small as a marble to as large as a basketball and can be fractured or brecciated (broken up). The internal bands can be translucent or opaque and of a variety of color intensities. Names for the various types of agates are usually self-descriptive: moss, plume or cloud, flame, banded, fortification, lace, eye, tube, and botryoidal are some of the common terms used to describe the textures, mottling, appearance, and structures within agates. Fortification: colored bands are tight, angular, of equal thickness, and repetitive, which give the appearance of a layered wall with sharp boundaries, much like a fort or tepee as seen in an aerial view. ky agate.indd 5 7/27/13 4:47 PM 6 Kentucky Agate Banded: color bands are similar to fortification, except that they are smooth and curvilinear rather than angular. Flame or fire: wispy or flamelike extensions with intensely hued red or orange, well defined or mottled. ky agate.indd 6 7/27/13 4:47 PM Lace: delicate and finely banded, usually white to gray in color, with sharply distinct layering; sometimes called crazy lace. Plume or cloud: nebulous, billowy extensions with poorly defined boundaries; it may be botryoidal (like a bunch of grapes or kidneyshaped ), or resemble flowers or clouds. Types of Agates 7 ky agate.indd 7 7/27/13 4:47 PM 8 Kentucky Agate Tube: distinct, uniform, fingerlike structures of various shapes and colors transecting the agate; the structures may appear stalactitic or stalagmitic, as shown in the lower portion of the photograph. Moss or dendritic: the internal agate texture is irregularly shaped, mottled, branching, or tree shaped; it may look like moss on a rock or the branches of a tree. ky agate.indd 8 7/27/13 4:47 PM Types of Agates 9 Eye: smaller, rounded fortification-type agates that look like eyes when cut perpendicular to the tubular shapes. Uruguay-type or watermark banding: parallel layered markings or bands result from evaporation of water levels and precipitation of residual silica minerals. ky agate.indd 9 7/27/13 4:47 PM 10 Kentucky Agate Sagenite: an agate containing thin, acicular or needlelike crystal inclusions radiating from a central point or points. ky agate.indd 10 7/27/13 4:47 PM ...