FMF - Friends of Minerals Forum, discussion and message board

[cascaillou]

These forms of silica can indeed show very close appearance , and some even share very similar composition, so we discussed their differences on a gemological forum and I thought it might be interesting to share it here. Here are a few definitions:

Chalcedony is a mineral, usually defined as a fibrous cryptocrystalline variety of quartz but more recently it has been shown that much chalcedony is a mixture with moganite (moganite is a monoclinic form of silica). It is transluscent to opaque. Iron-containing chalcedony can be heated to improve color.

Flint is a type of chert (but forming in a somewhat different environnement), so let's consider flint and chert are the same kind of sedimentary rock which is made of a mixture of microscopic quartz grains or microscopic fibrous chalcedony grains, mixed with moganite and some opal-CT, and coloured by metal oxides. Flint and chert can be transluscent to opaque. They can be heated to improve color.

Jasper is a rock (which is either volcanic or sedimentary), which can be defined as a dense opaque microcrystalline variety of quartz (or more strictly, chalcedony), coloured by metal oxides, clays can be present in the mix. It is related to chert/flint but forms in different environnements.

Quartzite is a rock made of merged quartz grains forming a tough interlocked mass, it is either metamorphic (formed through recrystallisation of sandstone, cherts, radiolarites) or sedimentary (formed through diagenetic cementation of sandstone). The metamorphic quartzite can be transluscent to opaque, it has been used as an imitation material (for chalcedony, jades..).

Chalcedony, flint/chert, jasper and quartzite can all be dyed.

Also, do not confuse any of these with what is called massive quartz (= non crystallised macrocrystalline quartz), which can be transparent, transluscent or opaque.


So...while a polished quartzite bead can look similar to chalcedony, yet it isn't too hard to separate from it under the microscope (due to quartzite granular structure).
Flint/chert often have a characteristic appearance, however there might still be some borderline cases making it harder to separate transluscent flint/chert versus chalcedony, or opaque flint/chert versus jasper (especially as polished stones).

Transparent flint:
https://images.worthpoint.com/files/pb/523/3493/223.jpg
(link normalized by FMF)

PS: the term 'polycrystalline' can be divided in two categories: microcrystalline (composed of small crystals that can be individually observed with a microscope) and cryptocrystalline (composed of such small crystals that a microscope cannot distinguish individual crystals)