19 August 2019

Botryoidal chalcedony

Chalcedony ( /kælˈsɛdəni/) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. The color of chalcedony sold commercially is often enhanced by dyeing or heating.

The name chalcedony comes from the Latin chalcedonius (alternatively spelled calchedonius). The name appears in Pliny the Elder's Naturalis Historia as a term for a translucid kind of Jaspis. The name is probably derived from the town Chalcedon in Asia Minor. The Greek word khalkedon (χαλκηδών) also appears in the Book of Revelation (Apc 21,19). It is a hapax legomenon found nowhere else, so it is hard to tell whether the precious gem mentioned in the Bible is the same mineral known by this name today.
Image viaI rather suspect this specimen has been dyed.  It was described as "natural" at the link,  but the word may have applied to the structural formation rather than the color.

Addendum:  A tip of the blogging cap (and my rockhounding sunhat) to an anonymous reader who found a discussion of this "grape" chalcedony in a mindat discussion thread.  Apparently the color can be natural, and it may be appropriate to consider this a form of amethyst! 


  1. There may be something amiss with me, I saw the pic above and thought 'Ooh, someone has dyed some glera grapes !' and then I thought 'Yum, prosecco !' and then that made me hungry because Italian shampagne goes with pizza (obviously), and then I got sad because its only twenty past eight a.m. and I think there may be laws about eating pizza for breakfast.
    Don't blame me, I didn't vote for Trump.

  2. The purple color seems to be legitimate as per this discussion on mindat https://www.mindat.org/mesg-385869.html (although of course it may have been artificially enhanced in these samples or digitally in the image).

  3. the only law (really a gustatory restriction) about pizza for breakfast is: you cannot have pizza with coffee.



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