Micro-World Gems & Gemology, Summer 2016, Vol. 52, No. 2

“Pond Life” Orbicular Chalcedony

“Pond Life” orbicular chalcedony
Figure 1. “Pond Life,” a 50.95 ct orbicular chalcedony cabochon measuring
48.5 × 27.0 × 5.5 mm, evokes a pond. The “frog eggs” are actually iron-containing
inclusions surrounded by the host’s concentric growth. Photo by Elise A. Skalwold.

The seemingly infinite combination of growth features and inclusions seen in microcrystalline quartz fires the imagination, often evoking visual metaphors (see “Aurora Iris Agate” in this column). Such is the case with the orbicular chalcedony seen in figure 1. From a piece of non-descript tumbling rough purchased in 2011, Paul Stalker (Stones by Stalkers, Tioga, Pennsylvania) delighted in creating what he christened “Pond Life,” as the 50.95 ct piece’s polished appearance resembles frog eggs within a pond.

Exploring the interior of such microcrystalline varieties of jaspers and agates can be just as fascinating as exploring inclusions within single-crystal quartzes, though these opportunities may be overlooked when dazzled by complex macro features. Orbicular chalcedonies such as this ocean jasper are particularly interesting. Here, iron-containing inclusions such as limonite, goethite, and hematite are surrounded by the concentric growth of the host material, which displays the unique fibrous texture found in some types of chalcedony (figure 2, left). Bundles of fibers composed of crystallites are combined with mutually complex optical orientations, giving rise to the eye-visible effect. While beautiful in transmitted, reflected, and polarized light, the addition of various contrast filters can dramatically enhance the details of these subtle growth features, making them easier to study, as well as creating stunning images of a specimen’s inner world (figure 2, right). For more on advanced filtering techniques, see Fall 2015 Micro-World, pp. 328–329.

Illuminated growth textures
Figure 2. The use of a red contrast filter with transmitted light (left) dramatizes growth textures, making them easier to study than with unfiltered transmitted light (right) while creating an aesthetically pleasing image. Photomicrograph by John I. Koivula; field of view 4.0 mm.

Elise A. Skalwold, an accredited senior gemologist of the Accredited Gemologists Association (AGA), is involved in curating and research at Cornell University in Ithaca, New York.