Tanzanite - Azotic Coating and D - Block Origin

 
The D Block term was used because the best tanzanite supposedly came from that dig.
Actually, wherever they hit the seam, they got great material. I can remember when the smallest piece I could get was over 10 cts., with trays of 20 ct. + material, huge rounds looking like acorns with the deep pavilions they cut them in. The costs then would still easily give you your keystone profit now, despite a 20% drop in value from its highest price.
Azotic coating will give you greater color saturation at the edges than the center, opposite what an untreated gem does.
Under a microscope, put the gem in the cut off bottom of a dixie cup with water just up to the girdle. The difference from eye-balling it will be stunning.
The "blue-green tanzanite, untreated" piece I tested was colorless, table down in the water. I turned it over and the color showed only on the facets above the girdle and the table was still colorless.
If you see any tubules of color filling inclusions, it has been lattice diffused. This process can take colorless rough and subject it to enough heat and pressure that introducing gaseous metals causes the rough to take on color. Crap material to gem material in the lab. A flood of no reserve bids, consistent color saturation and larger gems on e-bay from vendors who are power sellers you never heard of before is a dead give away. That's the deal with all this blue-green tanzanite; cheap because a lab used azotic coating to get color where there was none.
1 ct., emerald cut, VS and $9.99 to open and there's so much, mine was the only bid.
Please. Chinese vendors have pages of something the knowledgeable know was hard to come by and never cheap, but it sells and degrades the industry with every false sale of "untreated" gems.
Some of the faux material was even touted as "D block" origin. Insult to injury. 

It sounds like the standard immersion cell examination will reveal that classic color saturation near the facet junctions. I have not seen any lattice diffused goods yet (other than in the GIA NYC LAB), but I know they are out there in quantity. Hope the immersion exam will show that as well...

some coated stones will show a weird iridescence across the PAVILION when you rock/roll it under a light source -- especially some of the coated beryl/topaz/quartz materials, but also when examining stones such as fancy-color DIAMOND. Some it shows up better in diffused light, but some find it easier to see when viewed under more focused light, moving the light around the surface of the stone.  

Sapphires - Synthetic or Real

With scientific advancements comes the potential to run into more and more convincing synthetic stones.
How can you tell the difference?
The most effective way to tell the difference is in their Refractive Index of the natural stone. Also all stones formed from the earth will have some inclusions. 
Specific Gravity is not helpful in determining a synthetic from a natural stone.




3 simple tools together -- Loupe, Chelsea filter, and Dichroscope -- which are also useful in the field. And if you have a portable LW/SW UV lamp, that's also an important tool. For example, garnet is single-refracting and will show no dichroism (with a dichroscope, a dichroic stone will show one color in one window, and a different color or different "shade" of color in the other window of this small instrument). So this will immediately separate a garnet, glass or synthetic SPINEL imitating another gem from ruby, sapphire or emerald. In terms of tsavorite, when viewed through the Chelsea filter, tsavorite will appear reddish, but green glass or syn green spinel will NOT. Tsavorite is SINGLE refracting so it will show the same color in each window of the dichroscope, but some of its imposters (syn corundum) are double refracting and show dichroism, but in any event, those stones do not go red through the Chelsea filter. And then, the loupe will show inclusions in tsavorite that you will never see in glass, syn spinel, and so on. Natural, untreated blue sapphire will usually show no fluorescent reaction (under either wavelength) whereas treated-blue sapphire usually shows a blue/white reaction and synthetic blue sapphire will have a stronger reaction, and even stronger under short-wave. And so on. There is never just one test that will tell you what you need to know....it's a matter of collecting the "clues" from information provided by at least a couple of tests, and putting the pieces of the puzzle together.
Do you have my book "Gem Identification Made Easy" (published by Gemstone Press -- www.gemstonepress.com)? I talk about how to use a variety of instruments, and explain how valuable small, portable tools can be when you know how to use them PROPERLY, and when you use them together to collect the "clues" you need.