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.
The Carlsbad and New York laboratories and schools will be closed on Monday, May 28 in observance of the Memorial Day holiday. These locations will resume regular business hours on Tuesday, May 29.