LAB NOTES - World Gemological Institute
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LAB NOTES

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NEWS & ARTICLES

Ruby Glass -Composites

 

The association between ruby and glass has been documented by leading authority in the field since the Eighties.

Until the 2007 glass was involved only in the heating treatment for rubies with many fractures, cavities and a colour potential; indeed, at high temperatures, it acts as a catalyst and as a filler.During this year, different kinds of samples have been identified as composite of glass and ruby.

In these cases, glass was no longer used only as filler but also as an adhesive. Pieces of rubies were held together by it.

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The pictures show a ruby-glass composite. We can observe a disorderly web of fractures that goes form the surface within the whole stone.

The material in the fractures has a different lustre and texture, and scattered gas bubbles are trapped within.

This filler may be unstable at high temperatures like the bench jeweller torch; that is why special care should be taken when repairing or mounting jewellery with this kind of stones. Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

Flux – Grown Synthetic Ruby

 

A synthetic gemstone is a manufactured or man-made material that has a natural counterpart. It has the same chemical composition and crystal structure, and all the physical and optical properties of the natural gem. There are different processes to produce synthetics; in our laboratory, a few days ago we came across this flux- grown synthetic ruby.

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The flux-growth method can be explained easily as a solution of salt and water. Flux is a solid material that acts as a catalyst in the process, it helps to dissolve all the chemical elements needed for the gemstone to grow.

 

As the solution cools, crystals start growing, it is a process that requires around a year and it is very expensive. The key for separating natural from flux-grown synthetic ruby is magnification. The picture shows what we call ‘wispy veils’: whitish fingerprints going into each other disorderly. This flux remnants can be also high-relief, coarse, brownish, yellow to orange, drippy, tubular or icicle-like.  Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

Emeralds

 

The Emerald formation is a complex process and it takes place in an exceptionally turbulent environment where rapid changes of growth conditions are common.

It is also peculiar, and remarkably different from the other varieties of the Beryl Species: Aquamarine, Heliodor and Morganite.

 

Emeralds form because of metamorphism (pre-existent rocks are modified by raising temperature and pressure) or hydrothermal process (hot, chemically rich, water solution that forms minerals as it cools in rock veins) or both.

 

This kind of formation causes internal stresses and fractures in the mineral – for that reason fracture filling is a common treatment in the trade. Today there are many different types of filler: oils, resins or compounds, both natural and chemical.

It is crucial to disclose any treatment and the amount of it: minor, moderate or significant; not only because the value changes dramatically but also because the stone could break while it is being set in a jewellery piece or during successive repairs.

 

It is also important to disclose the clarity, since the heat from the jewellers’ torch, chemicals, sudden changes of temperature, can permanently damage the gem, whether if it is filled or not.

 

Even though emeralds have a lot of different type of inclusions that vary on the geology of individual deposits, the geographical origin of a gem is not always possible to determine, as many deposits have the same or similar geology, therefore the inclusions can be similar.

 

The salient examples are the well-known “three-phase” inclusions (liquid, solid and gas together), that until recently were believed to be found only in Colombian emeralds, but a scientific paper from GIA has shown that they can also be found in gems from Zambia, Afghanistan and China.

Thus, the geographical origin is an “expert opinion” based on the observation of intact inclusions and on the results of tests.

 

In the pictures below are shown some natural inclusions, from left and from top to the bottom: two-phase inclusions with a rectangular shape typical of Brazil, hollow tube of amphibole typical of Zimbabwe, Zambia and Russia and mica platelets typical of all the metamorphic emeralds as Zimbabwe and Russia.

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The pictures below show different views of a fracture filled Emerald.

 

From left to right and top to bottom: the diffused light is ideal to gain the whole vision of the stone, the outline of a fracture and an obvious difference in lustre and texture within the fractures, the filler has a shape “branch-like” because it does not occupy evenly all the space in the fracture.  Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

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Understanding Fluorescence

 

Let’s start defining what is this phenomenon:

Fluorescence is the visible light some diamonds emit when they are exposed to UV rays, and on WGI reports it refers to the intensity of the diamond’s reaction to Long-Wave UV which is an essential part of daylight.

The 25 – 35 % of diamonds exhibit fluorescence* and in 95 % of the specimen the colour seen is blue but can be any colour.

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Aurora Pyramid of Hope, Natural History Museum in London, shows 296 naturally coloured diamonds and how they change under Long Wave UV light.

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Picture of an antique and a modern piece of jewellery as seen under the Long Wave UV light, they show a great variety of colour and intensity of fluorescence.

 

In the trade Fluorescence is perceived as a negative property although, when a medium to strong intensity is present the diamond looks whiter and brighter in daylight. This means it can appear one or two colours whiter.

 

Only in extremely rare case when the fluorescence is very strong, diamonds could appear hazy or milky.

Hence Fluorescence is just a characteristic of diamonds, like the chemical composition or crystal structure, and not a given grade, plus it does not affect durability, clarity or value. Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

 

*https://4cs.gia.edu/en-us/blog/understanding-diamond-fluorescence/

Fracture Filling in diamonds

 

Fracture filling is a common treatment for diamonds to enhance the clarity but, being not permanent it only improves the appearance.

 

What is it exactly?

It is a molten glass filler that has a Refractive Index (RI) very close to the diamond’s RI, it means that the feathers/fractures are less visible.

As with every other treatment it is important to disclose: it affects the value; deep fractures, even if disguised, can affect the durability and, if the diamond is mounted in a jewellery piece, the temperature of the jeweller’s torch or sudden changes of temperature can damage the filler or cause the filler in the fractures to expand.

 

Which are the visual indications? And how is possible to detect them?

In gemmology what we call “flash-effect colours” are flashes of blue/violet or yellow/orange that occur within the fractures.

Sometimes the amount of fractures is so high that is possible to see these flashes with naked eyes through the table of the diamond, but usually, it requires a thorough inspection with the microscope or at least with the 10X Loupe.

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The best way to see fracture filling is through the pavilion, holding the diamond table to culet.

 

Why is it necessary to submit your diamond to an independent Laboratory?

Because the “flash-effect” can be mistaken for the natural iridescent colour seen in fractures, when the air interacts with the light.

Below from left to right a fracture filled diamond and an untreated one. Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

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Rough Diamond Simulants

Some days are more exciting than others and this was one of them, feeling like detectives in our lab.

We received two rough stones, octahedral shape, labelled as “rough diamonds”.

Something caught our attention: on the octahedral sides, there were big triangular marks that pointed at the same direction as the facets.

We can see from the pictures below these marks are obviously carved, the lustre is much lower than the one of a diamond and the edges of the marks are not sharp proving a lower hardness.

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The pictures below show genuine trigons: crystal structure’s growth marks

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In the second rough there were visible many features that proved its different species.

In the pictures below we can see many two-phase inclusions (gas and liquid together) typical of the hydrothermal formation that implicate a much lower temperature and pressure than the ones needed in diamond’s growth.

 

These inclusions are a hunch that the specimen can be a Beryl, a Quartz or a Topaz.

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Through other testes we eventually identified the two roughs as the first Colourless Sapphire and the second Colourless Topaz.

 

In this picture the Colourless Sapphire seen under the polariscope with conoscope shows a typical uniaxial optic figure that remind a cross (concentric circles called isochromes and two L shapes isogyres). Imagery and text by Fanny Raponi (G.G GIA) WGI Head Gemologist.

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