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[cascaillou]

First let's make a difference between quartz (macrocrystalline) and chalcedony (cryptocrystalline). Thus, these will be discussed separately.


1)QUARTZ:
This is about quartz and its varieties (including rock crystal, milky quartz, smoky quartz, morion, rose quartz, amethyst, ametrine, citrine, prasiolite, blue quartz, tiger-eye quartz, aventurine quartz, quartzite, etc.)

QUARTZ TREATMENTS:

Keeping in mind that several different treatments might be applied to a single stone...

Irradiation treatments (results will depends on the nature of the starting material):
-colorless and pale colors TO smoky or morion, or sometimes greenish yellow. This is reversible by heat.
-yellow or green TO amethyst. This is reversible by heat. But obviously it is not suitable to turn natural citrine or prasiolite which are quite rare into common amethyst.
-colorless or pale rose TO rose, or strawberry red. This is reversible by heat.
-amethyst TO slightly darker amethyst. This is reversible by heat.

Heat treatments (results will depends on the nature of the starting material):
-smoky TO lighter smoky, TO brownish-yellow, TO greenish yellow, TO green/blue-green/blue (rare change), TO colorless. This is reversible by irradiation.
-amethyst TO paler amethyst, TO yellow or orange (citrine) or bicolor amethyst-citrine, TO colorless or milky. This is sometimes reversible by irradiation.
-amethyst TO paler amethyst, TO colorless or green. This is sometimes reversible by irradiation.
-rose TO lighter rose TO colorless. This is sometimes reversible by irradiation. But obviously it is not suitable to turn rose quartz into mundane colorless quartz.

Treatments associating both irradiation and heating (results will depends on the nature of the starting material):
-colorless or milky TO greenish-yellow, yellowish-green, bright yellow, yellowish-brown, golden orange (these would possibly be sold under commercial names such as 'lemon quartz', 'oro verde', 'cognac quartz'...)
-amethyst TO green, or rarely dark blue or purplish-blue
-intensify yellow and purple in ametrine

Inclusions related heat treatments:
-quartz with a bluish hue caused by fine rutile inclusions or fine cracks can be heated to impart it with a more violet hue, although heating sometimes simply remove the blue hue making the quartz colorless. So-called "sunset quartz" can also be heated to improve the orange color imparted by fine needle inclusions. The asterism present in some rose quartz (which is caused by fine rutile needles) might also be modified by heating.
-quartz can be quench crackled to get 'iris quartz', or to get a milky 'schiller' (for imitating moonstone). Quartz can also be quench crackled simply to allow subsequent dyeing treatment.
-quartz with a yellow to brown color caused by inclusions of iron hydroxides such as limonite/goethite (for instance in tiger-eye) can be heated to get a brownish-orange, red-brown or red color.

Other treatments (fracture filling, dyeing, coatings, etc...):
-fracture filling with colourless oil or resin (of close refractive index, such as Opticon): makes cracks invisible (thus improving clarity of the stone). Glass filling is also possible, although less likely.
-dyeing of crystaline quartz (any color is possible): fractures are filled with a dye (adds color) or with a coloured oil or resin (adds color and improves clarity). Usually, dyed quartz was first quench crackled (so to produce a network of cracks all through the stone).
-impregnation of quartzite: quartzite (which is porous) is permeated either with a dye or a coloured oil or resin (any color is possible). Such dyed quartzite has been used to imitate other gemstones (such as chrysoprase, lapis-lazuli and jade). In aventurine (which is a variety of quartzite), colorless or coloured impregnation has been used to improve transluscency and color.
-surface coating (any color is possible) by a coloured varnish, coloured polymer resin, or thin coloured plastic film, or by thin metal film deposition (the later process can also produce iridescent/opalescent 'aqua aura' quartz).
Let's note that in faceted stones, the coating might be either applied all over the stone, or only on the girdle, or only on the pavilion.
-bleaching: tiger-eye being porous, bleaching can be applied so to lighten its colour, or so to turn it into grayish cat's eye. Quartzite which is also a porous material could also be bleached.
-white quartz which fractures have been filled (under pressure) with molten gold or silver (simulates gold-bearing or silver-bearing quartz).
-drilling holes in the stone to create fake inclusions (for instance holes filled with coloured mineral powder and resin, or hole filled with a liquid + small crystal and sealed with resin so to imitate a 3-phase inclusion)
-dyeing (for instance bright red) of surface reaching needles in rutilated quartz
-patterns (i.e. words, shapes...) can be laser engraved inside of quartz
-synthetic overgrowth: damaged natural crystals can be healed via hydrothermal process (natural quartz overgrown by synthetic hydrothermal quartz, either colorless or coloured).
-inclusions could also be embedded through synthetic quartz overgrowth. Here's an article about garnets inclusions and two-phase inclusions experimentally embedded in synthetic quartz: https://www.gia.edu/gems-gemology/winter-2016-microworld-synthetic-quartz-designer-inclusion-specimen
-specimen repairs: damaged quartz crystals can be repaired with glue. Crystal faces can be polished or even re-cut. Accidental fractures can be treated with hydrofluoric acid (dangerous!) to impart these with a more natural look. Crystals (either natural or synthetic) can be glued in natural or reconstituted matrix.

Treatments specific to cut stones:
-composites stones (doublet or triplet): this only apply to faceted stones. For instance, a triplet consisting of a quartz pavilion and a quartz crown assembled by a thin layer of orange or purple enamel so to imitate citrine or amethyst. Or a doublet consisting of a coloured plastic or glass pavilion assembled to a quartz crown. Or a doublet consisting of a quartz pavilion assembled to a tourmaline or rutile included quartz crown. Or a triplet consisting of a thin colorless or coloured transparent film printed or engraved with a "star" or "dendritic" pattern sandwiched between a quartz pavilion and a quartz crown (so to imitate asteriated or dendritic quartz). Also, a doublet cabochon consisting of a tiny piece of garnet sandwiched between two pieces of quartz to simulate an inclusion (the garnet was placed inside a tiny cavity which had been drilled and filled with glue). Also reported a doublet consisting of a white ceramic pavilion assembled to a gold-bearing white quartz crown. Etc. This being said, quartz based doublet and triplet stones are often made so to simulate more valuable gemstones than quartz (emerald, corundum, etc.)
-foil backing: this only apply to set stones. A reflective foil, either coloured or not, is applied on the back of the set stone (this intensifies luminosity, and also color if the foil is coloured. With asteriated rose quartz, the increase in luminosity would help intensify the asterism)

SYNTHETIC QUARTZ:
Quartz is synthesized via hydrothermal process. Any color is possible (colorless, pink, purple, yellow, orange, red, green, yellow-green, blue, blue-green, smoky, bicolor ametrine...). Let's note that synthetic quartz may also have been treated (i.e. to modify its color). Here's a link with with many pictures of synthetic hydrothermal quartz crystals and clusters (a few natural quartz crystals are also illustrated for comparison purposes): https://www.mineral-forum.com/message-board/viewtopic.php?t=4243&sid=979e2281ab57558fcab9674c1ba2d5fa

QUARTZ SIMULANTS:
-artificial: glass, synthetic spinel, or doublets (faceted) made from these.
As a side note, glass with inclusions (i.e. colour patterns, foreign materials embedded in glass, or devitrification figures) could be passed off as included-quartz.
-natural: fluorite (for amethyst), apatite...


2)CHALCEDONY:
This is about chalcedony and its varieties (including blue chalcedony, agates, chrysoprase, carnelian, onyx, sard, sardonyx, quartzine, plasma, jasper, flint and chert...).

CHALCEDONY TREATMENTS:

Keeping in mind that several different treatments might be applied to a single stone...

Heat treatments:
-pale colors (yellowish, orangish, brownish) imparted by microscopic iron hydroxides inclusions can be heated TO rich brown, orange or red color. This is not reversible.
-purplish "damsonite" (from Arizona) can be heated TO orange.
-pale colors (bluish, grayish...) can be heated TO milky white (this treatment is usually a preliminary step to a subsequent dyeing treatment). This is not reversible.
-brown sard can be heated to lighten its color.

Irradiation treatments:
-irradiation of chalcedony could darken the stone. This would be reversible by heat.

Other treatments:
-bleaching: staining caused by oxides/hydroxides could be bleached by permeating the porous chalcedony with chemicals (i.e. acids, bleach, etc.)
-colorless impregnation (the stone is permeated with a colorless oil or resin): reveals the original color of the stone and improves transluscency.
-colored impregnation (the stone is permeated with a coloured oil or resin): adds color (any color is possible) and improves transluscency.
-dyeing (the stone is permeated with a dye): adds color (any color is possible).
This will allow to imitate coloured chalcedony varieties (green for chrysoprase, orange for carnelian, etc.) or other gemstones (green dyed chalcedony for jade, blue dyed jasper for lapis-lazuli, etc.).
Samples that absorb dye unevenly to produce banded layers would imitate banded onyx or banded sardonyx.
Dyeing can also be applied more artistically so to imitate patterns encountered in naturally included chalcedony, for instance dendritic patterns painted with silver salts.
In landscape agate, the existing pattern can be accentuated with several shades of dye (blue and green notably).
-surface coating (any color is possible) by a coloured varnish, coloured polymer resin, thin coloured plastic film, or by thin metal film deposition (the later process can also produce iridescence/opalescence).
Let's note that in cabbed stones, the coating might be either applied all over the stone, or only on the base of the cabochon.
-a superficial network pattern of white veins can be produced by a process associating quench crackling and the use of a flux or bleaching agent.
-adding dendritic inclusions: the stone is permeated with a tin or copper salt, and dendrites of tin or copper are formed by electrolysis or by chemical precipitation.

Treatments specific to cut stones:
-composites stones (doublet or tiplet): this only apply to faceted stones. For instance, a triplet consisting of a chalcedony base and a chalcedony dome assembled by a thin layer of green enamel (could imitate chrysoprase). Or a doublet consisting of a coloured plastic or glass base assembled to a chalcedony dome. Or a triplet consisting of a thin colorless or coloured transparent film printed or engraved with a "dendritic" pattern sandwiched between a chalcedony base and a chalcedony dome (so to imitate dendritic agate). Or a doublet consiting of an amazonite base assembled to a chalcedony dome. Two or three layers of chalcedony of different colors can be assembled and then cut into cameos or intaglios (so to get a multicoloured pattern). Also reported a doublet consisting of an agate base assembled to a quartz dome. Etc. This being said, chalcedony based doublet or triplet stones are often made so to simulate more valuable gemstones than chalcedony varieties (such as jade, emerald..).
-composite mosaic: small pieces of chalcedony (often dyed) are assembled alltogether by a polymer resin. The obtained mosaic is then cabbed or cut into decorative objects.
-foil backing: this only apply to set stones. A reflective foil, either coloured or not, is applied on the back of the set stone (this intensifies luminosity, and also color if the foil is coloured)

SYNTHETIC:
Chalcedony has been synthesized experimentally but has not been commercialized.

CHALCEDONY SIMULANTS:
-artificial: plastic, glass, glass paste (aka 'pate de verre'), ceramics, glass-ceramics, etc.
-natural: flint/chert (which can be heated or dyed to improve color), dyed quartzite, dyed marble, dyed alabaster, dyed serpentine, Etc.
Rhyolite, dyed magnesite and dyed howlite may imitate jasper. Limestone-onyx and alabaster shouldn't be mistaken for agate. Various green transluscent stones could possibly be mistaken for chrysoprase (aventurine quartz for instance).
-jasper could also possibly be imitated by a composite material (i.e. mineral powder and pigments/dyes mixed to a binder material such as resin or cement)

[Pete Richards]

This is a very valuable summary of common treatments that may make our preferred natural specimens or gems look different (and more attractive, perhaps) than they should. It is something that we should keep in mind as we look at potential mineral/gem purchases. Thanks, cascaillou!

[cascaillou]

In the first place, it is indeed important to know which treatments apply to which minerals and with which results. But then comes the hard part: detection of treatments.

While detection of some treatments might be more or less easy (i.e. using magnification and basic gemological testing), detection of other treatments might require lab analysis. Unfortunately, in some cases, irradiation or heat treatments are impossible to prove or disprove even with lab analysis.

If one is interested in treatments and their detection I'll recommend the following book:
'Gemstone enhancement', by Kurt Nassau, second edition (1994)
Unfortunately not published anymore thus hard to get your hands on (especially the updated second edition).

Also, lots of documentation about treatments, synthetics and simulants is available from gemological websites, and notably from the websites of major gem labs.

[cascaillou]

Updated.