Chalcedony

Chalcedony is a quartz variety and in general denotes all quartzes of micro- and cryptocrystalline structure.
In microcrystalline minerals crystals are discernible by 10x loupe, whereas in cryptocrystalline minerals it takes a microscope.

In modern mineralogy the term chalcedony is used more specifically and denotes aggregates of quartz consisting of parallelly grown fibrous crystals of microscopic and sub-microscopic size. According to the orientation of the fibres one distinguishes "length-fast" and "length-slow" chalcedony, the so-called quartzine. In length-fast chalcedony the axis of higher R.I. (refractive index) lies at right angles to the direction of the fibres. In quartzine this axis is parallel to the fibres.
However, mineralogists disagree on whether quartzine is indeed a chalcedony or wheter it is a variety of fibrous quartz on its own.

Origin of name: from Greek χαλκηδών Chalkēdṓn, an antique seaport on the Bosporus, today Turkish Kadıköy, a district of Istanbul. First mentioned about 77 A. D. by Pliny the Elder, who names a calchedon stone in a list of translucent jaspers in his monumental work Naturalis Historia.

Can be confused with: due to the manifold appearances chalcedony can be confused with a plethora of other minerals e. g. other quartzes (jasper and agate), calcite and aragonite, aventurine, jade, rhyolite and many more

Localities: on all continents including Antarctica; mindat lists almost 6500 localities!