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EducationThe Shadows of Black Diamonds

13th Mar 2016 15:09 UTCcascaillou

THE SHADOWS OF BLACK DIAMONDS

Let's share the documentation I have been gathering...

1)MONOCRYSTALLINE DIAMOND:

1.1-Black appearance from inclusions:

Color from mineral inclusions:

In most cases, black diamonds owe their dark gray to black appearance to dark mineral inclusions (graphite, sulfides, magnetite, hematite...).

But let's make a difference beetween stones which only appear black due to numerous surface reaching cracks filled with dark materials (these are common), and those which inclusions are fully embedded within the stone (these are more rare).

Color from unusual inclusions (rare):

*A faceted diamond with black appearance caused by dense dark brown hydrogen-related clouds extending throughout the stone was reported: http://www.gia.edu/gems-gemology/fall-2008-lab-notes

*A faceted diamond with black appearance caused by dark brown natural radiation staining along fractures was reported: http://www.gia.edu/gems-gemology/WN13-LN-black-diamond

1.2-Black appearance from very dark body color (rare):

*A faceted diamond almost appearing black but actually very dark purple (very rare) was reported, which color was attributed to hydrogen related defects: http://www.gia.edu/gems-gemology/summer-2009-lab-notes

*Diamond almost appearing black but actually very dark brown has also been reported (see AGL & GCAL article 'Inside Black Diamonds')

*Diamonds crystals with a very dark green 'skin' imparting these with an almost black appearance have been reported, which superficial color was caused by natural radiations from surrounding uranium rich rock. The dark color is only skin deep and would be removed when cutting the stone. (see AGL & GCAL article 'Inside Black Diamonds')

2)POLYCRYSTALLINE DIAMONDS:

It's important to make a difference between monocrystalline black diamond and polycrystalline black diamonds.

Polycrystalline varieties of diamond:

-Carbonado: massive aggregates, finely grainy compact structure, slightly porous. Very tough. Opaque, gray to black or brown, sometimes gray-green. Also note that SG is 2.9-3.5

-Bort (aka boart): grainy aggregates. Opaque to transluscent, dark color (olive, brownish, grayish, gray, black), without disctinct cleavage. (as a side note, 'framesite' refers to a very fine grained black bort, 'stewartite' refers to gray to black bort which is magnetic due to magnetite inclusions, and "hailstone bort' refers to a bort of zoned concentric structure)

-Ballas (aka shot bort): spherical to subspherical aggregates, fibrous diamond in compact, radiated or concentric structure. Transluscent to opaque (color is pale grayish, brownish, gray or black)

-'coated' diamond: monocrystalline diamond coated with a layer of transluscent to opaque fibrous polycrystalline diamond (which might appear yellowish, brownish, grayish, blackish). This coating is removed when cutting the stone.

In polycrystalline black diamonds, the color is caused by dark mineral inclusions (graphite, magnetite, hematite, sulfides..)

3)SOME PROPERTIES OF BLACK DIAMONDS:

Black color could occur amongst any diamond type (Ia, Ib, IIa, IIb)

Due to their mineral inclusions, black diamonds are often electrically conductive (unevenly), and sometimes showing magnetism.

Indeed, graphite is electrically conductive, while magnetism is found in magnetite and hematite for instance.

As a side note, beware that while a black diamond will test positive as diamond with a thermal tester, some diamond-moissanite testers discriminate moissanite on the basis of its electrical conductivity, thus such testers might also discriminate some black diamonds as well.

4)ONLY FRAUDS ARE FOREVER:

Note: detection of treatments, synthetics and simulants won't be discussed here, instead I'll recommend the read of the following articles:

-Gemlab article: 'Diamond-treatments, synthetic diamonds, diamond simulants and their detection' (by Thomas Hainschwang)

-AGL report: 'Inside Black Diamonds' (by Christopher P. Smith, Elizabeth Quinn Darenius and Sharrie Woodring Hand)

4.1-TREATMENTS:

*Irradiation treatment:

Such treatment can apply to any diamond Type, and works both for rough and cut stones. Irradiation treatments might sometimes be unstable to heat.

Irradiation of all diamond types will produce blue, green or blue-green color. Subsequent annealing will stabilize and/or modify the color. The color obtained after annealing depends on diamond Type (but let's note that in Type II irradiated diamonds, annealing would reverse the change).

Irradiation allows to produce diamonds of such a dark color that they appear black, but these aren't truly opaque and are actually very dark green, very dark blue, or sometimes with subsequent annealing very dark orange or brown-orange.

*Superficial ion-implantation:

This treatment is more adapted to cut stones. It is somewhat unstable to heat, and the color will be removed if the stone is re-cut. This treatment can be used to impart the stone with a superficial black color (electrically conductive)

It is hardly detectable out of the lab, but rarely encountered.

*Graphitisation treatment:

Such treatment is stable and can apply both to rough and cut stones.

Either high temperature treatment under a vacuum or HPHT treatment is used to turn low quality diamonds into dark gray to black diamonds (color from graphite inclusions). Detection of such treatment is problematic, even with lab analysis.

*DLC film:

This treatment applies to cut stones. It is stable but will be removed if the stone is re-cut. The treatment consists in applying a black and opaque DLC film over a low quality diamond.

DLC refers to 'diamond like carbon', which is amorphous carbon, almost as hard as diamond (around 9.5 Mohs). Such stone would test positive as diamond with a thermal tester, but DLC also shows very high electrical conductivity.

Detection not possible out of the lab (although any black diamond with such high electrical conductivity is suspicious and would thus require lab analysis).

*Dyeing:

Dyeing treatments are more likely to be found in cut stones, although dyed rough diamonds have been reported. They are unstable to abrasion and/or solvants.

Low quality diamonds which fracture had been filled with a black dye so to simulate black diamond have been reported.

*Glass-filling:

Glass-filling mostly applies to cut stones, although it has occasionally been reported in rough diamonds. It is not stable to heat. Fracture-filling with high RI colorless glass is usually meant as a clarity treatment, but glass-filling the fractures of a black diamond would still be possible (and why not with black glass?).

4.2-SYNTHETICS:

Synthetic HPHT (high pressure high temperature) diamonds and synthetic CVD (chemical vapour deposition) diamonds can be found in black color (either as very dark blue IIb synthetic diamond, or as synthetic diamond treated to black color).

Synthetic NPD (nano polycrystalline diamond, which is synthesized through an ultra-HPHT sintering process) has also been reported in black color (heavily included with graphite crystals).

As a side note, rough synthetic HPHT diamond crystals show a characteristic cuboctaedral shape with sharp edges and flat faces, which should limit the risk of confusion with natural diamond crystals. Rough CVD synthetic diamonds show a very flattened shape which is different from natural diamonds. And the rough of NPD is cylindrical.

4.3-SIMULANTS:

*Confusions: black glass, black onyx, hematite, ilmenite, black cassiterite, black titanite, blackish rutile, black garnet (melanite), black spinels and black synthetic spinel, hercynite, chromite, blackish zircon, black CZ, YIG, black YAG, black moissanite (actually very dark green), black strontium titanate, black synthetic corundum, silicon, hematine, boron carbide, etc...

As a side note, a CZ or moissanite could be coated with a DLC film or with a polycrystalline synthetic diamond film (CVD/PVD process).

Beware that simulants could be cut into the shape of a natural diamond crystal (with engraved striaes and trigons), or rounded to look like alluvial. Also keep in mind that some are naturally occuring as octahedral crystals (for instance black spinel group minerals, others such as cassiterite and zircon may show pseudo-octahedral shape).

*Sintered diamond: a black diamond simulant. Low quality diamond powder, sometimes associated to a metallic powder (Si, Fe, Ni), is sintered under high temperature and high pressure. Tests positive as diamond with a thermal tester.

Out of the lab, magnification (resolves individual grains) and fluorescence (fluorescent specs) may allow detection.

*Doublets: a black doublet would be a possibility (with either natural or synthetic diamond at the crown, or even with no diamond at all)

13th Mar 2016 15:46 UTCAlfredo Petrov Manager

This would be useful as an Article in Mindat, no? If you would like to consider submitting it as an "Article", rather than a mere messageboard post, go to your Mindat personal home page: click on "My home page", then on "My articles", then on "Create new article".

13th Mar 2016 16:14 UTCcascaillou

I'm using a pseudonym thus cannot post articles. For now, I post my articles directly in the forum.

13th Mar 2016 17:01 UTCPaul De Bondt Manager

Cascaillou, you already posted 2 articles. Why not this one ?

13th Mar 2016 17:08 UTCAlfredo Petrov Manager

Sorry, I'd forgotten about the pseudonym issue. Paul, Mindat has plans to soon revamp the Articles section into a more serious publishing venue, and then authors' names and contact info would have to be required, just as in Rocks & Minerals, Mineralogical Record, or American Mineralogist.

13th Mar 2016 17:24 UTCcascaillou

article updated.

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