Microscopic examination revealed a most unusual inclusion scene: All of the beads were full of hollow growth channels and nail-head spicules (wedge-shaped, liquid-filled growth channels terminated by an inclusion on one end). Nail-head spicules are characteristic inclusions in synthetic beryl and synthetic quartz, though similar-looking inclusions have been described in some natural stones. Thus, isolated inclusions of this type do not necessarily offer proof of synthetic origin. 



Nevertheless, the appearance of the nail-head spicules in these beads was typical for synthetic material, especially since the "heads" of the spicules contained "breadcrumb" inclusions, the most characteristic and common inclusion in synthetic quartz. This contributor, however, has never seen such a large number of these inclusions in any type of synthetic material. Each exhibited the typical wedge shape, and in most of them the liquid contained a gas bubble. 

The inclusions, hollow cavities and nail-head spicules were oriented parallel to the c-axis. In determining the optic axis direction, it was evident that none of the material was twinned -- unlike most natural citrine, which is created by heat-treating natural amethyst that commonly contains Brazil-law twinning. The large, hollow cavities likely represent oversized nail-head spicules that were either exposed by the polishing process or reached the surface during the growth process. In some of these very large cavities, the breadcrumb inclusion was found at the narrow end of the channel; in the others, it was absent. These features, like the smaller nail-head spicules, are probably attributed to rapid growth conditions.


Synthetics
 

 

Various methods and tests for discrimination of natural and artificial origin quartz and its coloured varieties were earlier published in a number of papers. However, a variety of technological possibilities are responsible for morphological characteristics and an internal growth structure in crystals. Such characteristics are also caused by their growth rates and constantly ad-mixed components. Identification features can completely exclude the unambiguous identification of an artificial origin of amethyst, citrine, ametrine, morion, smoky and colourless quartz after instrumental analysis. Tests proposed include optical and IR spectra. The discriminating featurexs consist in external morphology and an internal structure of natural crystals and their synthetic counterparts, including the presence of specific sectors and zones of growth, character and distribution of twins. Optical and IR spectra may be used for identification of natural and synthetic quartz only to a limited degree. The most reliable characteristics to distinguish between natural and synthetic quartz and its coloured varieties are fluid and solid inclusions. But in the stones of usually high-qualities they are absent.
 
The problem is increasing every year due to improved perfection of growth technologies. 

Key Separations, Suggestions for Testing and Evaluation
Aventurine Quartz

Aventurine quartz is a translucent form of quartzite (a metamorphic rock made up of interlocking quartz grains) containing tiny reflective or colored inclusions of hematite, mica, or other minerals. It is a microcrystalline aggregate that is usually sold as beads, carvings, or cabochons.
Reflections from bright green chrome mica (fuchsite) inclusions cause the glittery phenomenon called aventurescence in aventurine quartz. Depending on the nature of its inclusions, aventurine quartz can resemble aventurine feldspar, glass, jade, or jade substitutes. Magnification reveals these inclusions as tiny green disk-like platelets randomly distributed throughout the gem.
Aventurine quartz with green mica inclusions can be mistaken for nephrite in appearance. It can be used as a jade substitute and has been sold as “Regal jade” and “Indian jade.” It is easily separated from both jadeite and nephrite by its refractive index (R.I.) of 1.54 to 1.55 and specific gravity (S.G.) of 2.66, as well as by magnification.
The gem has a granular fracture and lacks the grid-like structure (caused by two intersecting cleavage directions) of amazonite microcline feldspar. Aventurine quartz has an R.I. close to chalcedony, but none of the green chalcedony varieties have chrome mica inclusions. Chalcedony has a slightly lower S.G. (2.60) and a conchoidal fracture with dull luster.
Pale to colorless quartzite is commonly dyed green to imitate various green gems such as jadeite. This material also lacks the chrome mica inclusions seen in the aventurine variety and shows dye concentrations. It may show a vague band in its visible spectrum around 630 to 660 nm due to the presence of dye.

Ametrine, Quartz Variety

Ametrine, also known as bolivianite, is a naturally occurring variety of quartz, containing both amethyst and citrine.

Ametrine is a very durable gemstone suited to a variety of jewellery uses.

Artists and experienced gemstone cutters are allowed to play with the two-tone singularity of ametrine yielding beautiful and unique cut gemstones.

Ametrine is a variety of quartz that contains both amethyst and citrine sectors in the same crystal. Both amethyst and citrine are coloured by small amounts of iron (approx. 40 parts per million). Amethyst color develops when iron-containing quartz is exposed to ionizing radiation. In nature, gamma rays from the decay of potassium-40 are the most likely source of ionizing radiation. The model currently accepted is that radiation oxidizes Fe3+ to Fe4+. There is still uncertainty about the site of the iron. Both interstitial sites in the c-axis channels, and the silicon tetrahedral sites have been proposed as the site of the amethyst center. Citrine colour is from Fe3+. The properties of the Fe3+ spectra suggest that the Fe3+ ions are aggregated and hydrated in clusters of unknown size.

Ametrine can be natural or heat-treated, however has heating this gemstone it become a common practice?
I do not believe that heat treatment is ever specified in this case. Several cutters here in the US were quite shocked when he heard about the heat treatment.
Bolivia is still the major supplier of the world’s ametrine.
The question is : How is heat treating done? Is it the mine/supplier who does it?

Synthetic ametrine is known to exist and has been documented in the gemmological literature. One somewhat suspicious sign is a sharp V-shaped border between the citrine and amethyst colours. Natural ametrine tends not to show this particular neat V-shape. Another suspicious sign is that, in rare cases, the cutter may have left a small colourless zone visible in the finished product (this is a remnant of the thin quartz slab used as a 'seed').

Both of these characteristics are visible in a photo of synthetic ametrine, shown here…
_http://www.faceters.com/rough/synthetics/quartz/ametrine_qtz.shtml_
…look in the centre of the crystal and you can see traces of a colourless transverse zone through the middle. Towards each end you can see the sharp V-shaped boundary between the citrine and the amethyst.

Here are natural colour zoning patterns, for comparison:
http://www.neweragems.com/gallery/medium/ROYAL_AMETRINE.jpg

Now, I have seen several references on the internet to ametrine being the result of heat-treatment. However, I regard these as dubious. Although the heat-treatment of amethyst to transform it into citrine is well-documented, I am not aware of any formal reference to the PARTIAL heat-treatment of a crystal in order to turn one half to citrine whilst keeping the other half as amethyst. In fact, some commentators on a gem-cutting internet forum have specifically cast doubt on these claims,
pointing out that there are obvious practical difficulties involved in only heating part of a crystal. How do you stop the heat from conducting throughout the crystal? How would you cool one side of the gem? How do you get a nice sharp delineation between the colours? Wouldn't the stone get damaged through thermal shock? Doesn't it sound like a rather labour-intensive and presumably expensive method? If it were a common procedure, why aren't the details of the process more widely known? Why
would you perform this procedure when synthetic and natural material are available? Why would people even bother to have developed hydrothermal synthetic ametrine in the first place if you could get this result from heat-treatment?

I therefore strongly suspect that the internet texts stating that ametrine is the result of heat-treatment are mere speculation. Natural untreated ametrine and synthetic hydrothermal ametrine are both well-known and I don't see any reason why these two sources couldn't account for all the ametrine on the market.

Synthetics 
Gem-quality synthetic ametrine has been produced commercially in Russia since 1994, by hydrothermal growth from alkaline solutions. Faceted synthetic ametrine has many similarities to its natural counterpart from Bolivia. For the most part, however, the synthetic ametrine obtained for this study could be identified by a combination of characteristics, including growth features such as twinning and color zoning. EDXRF chemical analyses revealed higher concentrations of K, Mn, Fe, and Zn than in natural ametrine. IR spectra of the synthetic citrine portions showed more intense absorption in the 3700-2500 cm-1.range compared to natural ametrine; the synthetic amethyst zones showed a weak diagnostic peak at 3543 cm-1.

Locality 
Bolivia
The only known significant commercial source of ametrine is the Anahi Mine, Santa Cruz, Bolivia, operated by Minerales y Metales del Oriente, S.R.L. and currently employs about 70 workers.
 
The mine owner, Ramiro Rivero,cuts part or all his gemstones in China.
Further more, there is also some ametrine rough available from Brazilian dealers as well as in India and even Bangkok.
Cutting shops and dealers from Idar Oberstein also offer top custom cut ametrine.
 
India
Minor amounts of citrine occur in some amethyst from Hyderabad, India. The 5 cm wide crystal contains only a minor amount of citrine colour. Citrine colour is found in few specimens.

 


Brazil
An occasional specimen of Brazilian bicoloured quartz has been found which contains both amethyst and citrine zones. It does not approach the quality of the Bolivian material.

Amethyst, Quartz Variety
Introduction and History
Amethyst is the most highly valued member of the Quartz mineral family, and it has Purple hues that vary from very pale to dark. Deep blue gems are rare and command higher prices but unlike other gems many actually prefer the more crystal clear gemstones to the richer, deeper colours.
The name is generally said to be derived from the Greek a, "not," and methuskein, "to intoxicate," expressing the old belief that the stone protected its owner from becoming inebriated. It was held that wine drunk out of a cup of amethyst would not intoxicate. However, the word may probably be a corruption of an Oriental name for the stone. 

Rose de France Amethyst is heat sensitive, excessive heat could turn the stone colourless, or it may produce a Citrine or green colour, whereas an abrupt temperature change may fracture the stone.

Colour and Cause Purple colouration which is caused by impurities of iron or manganese.
Degree of Transparency Transparent to translucent
Treatments 
Amethyst can be heat treated to improve the colour or change it to citrine. Not common. A new source of Brazillian amethyst is green.

Synthetics
 

Synthetic hydrothermally grown amethyst is plentiful and common. Most are probably mixed into rough and cut gemstone parcels and sold. This is not helped by the fact that differentiation between natural and synthetics represents a difficult to impossible task by normal gemmological procedures. Neither is it helped by the fact that amethyst is a low cost gemstone, and gemmological evaluation is most of the time not cost effective except in only larger gemstones.
Advanced spectroscopy, via Infra-Red spectroscopy, offers instant diagnostic differentiation in most cases; but such equipment is not readily available except to larger gemmological labs. 
The presence of absorption bands at 3680 ,3664 3630 unambiguously proves a synthetic orgin, but only for samples grown in near-neutral NH4F solutions. Conversely, there are no unambiguous diagnostic features in the IR spectra of the more commercially significant synthetic amethyst grown in alkaline K2CO3 solutions. Nevertheless,  previous investigators have found potential diagnostic value in absorption bands at 3595 and 3543 cm-1.  The 3595 band is found in the spectra of synthetic amethyst, it also is frequently absent from those of natural amethyst, it also is frequently absent from those of natural amethyst. The 3543 band is found in most synthetic amethysts grown in alkaline solutions but this band also is sometimes present in natural amethyst, so it provides only tentative evidence of synthetic origin. The unambiguous identification of natural vs. synthetic amethyst must be based on a combined examination of the IR spectra, internal growh structures(including twinning) and inclusions. (The 3543 cm-1 infrared absorption band I natural synthetic amethyst and its value in identification, Balitsky et al, Gems and Gemology, 2004, pg 146-61)

Origin
Pegmatitic dikes; hydrothermal veins
 
Locality 
Brazil(Rio Granda, Para), Madagascar, Zambia, Uruguay, Burma(Modern Myanmar), India, Canada, Mexico, Namibia, Russia(Siberia), Sri Lanka and the United States(Arizona)

Value Amethyst has lost much of its substantial value due to the discovery of extensive deposits in locations such as Brazil. Even high-quality examples are often sold in large unfinished slabs, or as geodes, in everyday locations. Value per carat in amethyst therefore, unlike many gems, does not rise exponentially with weight as it is readily available in large sizes; price depends almost entirely on colour. The "Siberian" deep purple with red and blue flashes commands the premium. As amethyst is plentiful, there is little reason to settle for stones with visible inclusions or inferior cutting.

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