I'd be very cautious about using rounded facet junctions as an
indicator of much of anything except poor polishing procedures. I
get to re-cut/re-polish lots of tanzanite and when they leave here,
they have sharp facet junctions and a mirror polish.
As a G.G. required to make gem identifications on a daily basis, I
am always cautious about every indication of a gem's identity.
Certain especially diamond, have characteristically sharp
facet junctions, while zoisite (tanzanite's species) does not.
Compare them and you will see what I am talking about. This is not an
indication of a cutters' skill or equipment. Rather, it is a natural
characteristic of zoisite.
What you call a mirror polish, gemologists call vitreous (Greek for
glass-like) luster. Gems with sharp facet junctions typically have
adamantine (diamond-like) luster. This is the main reason I never
use the fiber optic light when examining a diamond under the
microscope - the reflection from an adamantine stone can cause severe
retinal problems. I'm sure your stones' junctions are as sharp as
well-maintained, contemporary equipment can make them, but no
zoisite is capable of displaying junctions that are considered sharp
by gemological standards. And when the question of gem identity is
asked, gemological standards are required, not optional.
Rounded facet junctions, along with refractive index, specific
gravity, birefringence and other data are most certainly indications
of tanzanite. And as a Type I gem, it is typically very clean. So,
the lack of inclusions and rounded facet junctions that were
mentioned in the original post are absolutely some of the first
indicators of tanzanite, but they certainly aren't enough to go on by
themselves. When I said the word "indicator," I meant just that.
Indicator, not proof.
James S. Duncan, G.G.
James in SoFL