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.
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