Quartzine

A variety of Chalcedony

A fibrous variety of chalcedony. It is also called "length-slow chalcedony" and is usually intergrown with another, more common type of fibrous chalcedony, "length-fast chalcedony", that comprises most of the agate, chalcedony and flint specimen. Quartzine is more uncommon.

Quartzine "fibers" are made of tiny quartz crystals that are stacked along the c-axis (the long axis of the crystals).

It is not possible to identify quartzine with the naked eye, one needs a polarizing microscope to do that (which is also the reason for the odd names "length-slow" and "length-fast chalcedony" that refer to a special optical property of the chalcedony fibers).
However, the peculiar patterns seen in some chalcedony specimen, most notably so-called "feather agates", are caused by the intergrowth of quartzine with "ordinary" length-fast chalcedony (see photo).