Blue chalcedony cabochon (probable)

Smooth, polished round cabochon with pale blue to blue-gray color, translucent to semi-translucent body, and vitreous to waxy luster. Quartz hardness is about 7 on the Mohs scale; specific gravity is approximately 2.58–2.64. Chalcedony has a trigonal quartz structure, lacks prominent cleavage, and commonly shows conchoidal fracture. The image is somewhat blurred, so blue calcite, blue glass, or another pale-blue gemstone cannot be ruled out without testing.

Chalcedony forms when silica-rich groundwater deposits microscopic quartz fibers in cavities, fractures, and vesicles of volcanic rocks, often during relatively low-temperature hydrothermal alteration. It can also form as replacement material in sedimentary rocks. Chalcedony occurs in rocks ranging from ancient Precambrian formations to young volcanic deposits; an exact age cannot be determined from the photograph. The pictured piece has been cut and polished into a cabochon.

Used for cabochons, beads, carvings, ornamental objects, and inexpensive jewelry. Blue chalcedony is valued for its attractive color and translucency, though natural strongly saturated blue material is less common than gray, white, or lavender chalcedony.

Chalcedony is not a single crystal but an aggregate of extremely fine quartz crystals and sometimes minor moganite. Its blue color may result from microscopic inclusions or scattering effects. Strongly uniform, intensely blue material may be dyed, and some pale blue commercial cabochons sold as 'blue chalcedony' may actually be blue calcite, quartzite, or glass.

Common sources of chalcedony include Brazil, Uruguay, India, Madagascar, Namibia, Turkey, the United States, and Mexico. Field identification requires more than appearance: natural chalcedony should feel relatively hard, resist scratching by a steel file, and scratch ordinary window glass, while glass generally does not reliably scratch glass without damage. A refractive-index, specific-gravity, microscopic, or gemological test is preferable. Blue calcite would be much softer (Mohs 3) and would not scratch glass; man-made glass may show bubbles or flow lines.