I recently received the above collection of nice gemstone rough from a forum member on Tas-Prospecting – the member had fossicked these stones from the Weld River in Tasmania’s NE. The parcel contains a large smokey quartz, several water-worn rounded topaz crystals (approximately 3 – 4ct size each) and some sapphires. I am selecting and cutting any suitable stones for him. The front-most sapphire (around 2.2ct size) has a lovely blue tone as you can see below.
A geological interlude…
Corundum is widespread in Tasmania, mostly in the form of small sapphire grains in alluvial deposits. These are probably related to the occurrence of similar material throughout eastern Australia and South East Asia. The sapphire is thought to have been formed in syenite or similar rocks at great depth, and rapidly brought to the surface in pyroclastic eruptions of alkaline basalts, which are usually closely spatially associated.
Above extracted from Bottrill (1996) Corundum and Sapphire in Tasmania Tasmanian Geological Survey Record 1996/05
So, in a nutshell, the sapphires found in Weld River have been eroded out of the Tertiary alkaline basalts common in the local area, then concentrated in the small streams draining these basalts. The Tertiary alkaline basalts are the same rocks that weather to form the rich red-brown soils in the Scottsdale area, so productive for crops etc. These basalts in effect provided the transport mechanism for the sapphires, from their point of creation deep within the earth’s crust, to nearer the surface.
I will begin by cutting two topaz pieces – the piece on the right in the photo below has a slight aqua tone to it. Since both stones are relatively small, I will choose designs with fewer facets, but go for good light return.
Ok, smaller topaz on the right: Morph Square – Reflector. The topaz on the left will be a cushion cut (see below).
Also starting to have a look at the large ~ 32ct smokey quartz crystal for cutting purposes. Most of the stone has pretty good clarity, however a cloudy section will need to be trimmed (see below).
This smokey will make a lovely opposed bar cut gem, it has a very nice colour tone. Since the final length to width ratio of the rough will be about 1.50, the Jeff Graham Opposed Bar cut will fit the bill, if the ratio had been closer to 2.0, I might have cut it as a Marco Voltolini Htims Bar.
The two sets of parallel facets perpendicular to one another – one on the crown, and the other on the pavilion combine to make an attractive, flashy checkered effect in the cut below.
OK, back to the topaz. Ready to transfer cushion topaz in order to cut the crown.
Back to topaz number 2 (Morph Square – Reflector). Putting in the first crown facet
Ready for pre-polish. (Slight interlude and back to cutting again after some urgent geology work over last few days).
Pre and final polishing went OK, and the first two topaz’s are finally done. Reflectors are notoriously hard to photograph, a slight angle seems the go, as you can see from the image below.
Also starting to think about a design for the sapphire pictured at the start of this post. I’m thinking perhaps a Tic – Tac (90.6% light return). The design is selected to maximise the yield of the stone. A squarish shape is going to be the best idea from examining the stone in detail.
Back to the smokey quartz. I have re-oriented the smokey on the dop as I felt that I would get a slightly better yield by rotating the stone by 90 degrees.
I am starting to prefer dopping wax rather than cyanoacrylate, much easier to remove the stone when required by applying some heat.
The pavilion bar facets you can see above are only separated by a few degrees each, which helps to create the amazing flash effect seen when just moving the finished gem a little from side to side.
I have already pre-polished the bars (using diamond on a tin lap).
The Darkside Lap above is great for final polishing using oxides for quartz and a range of other gems, I am using cerium oxide on the Darkside (with water as a lubricant) on the smokey.
Ready to transfer so the crown (opposed) bars can be put in.
Let’s cut a sapphire now – doing the Tic – Tac cut.
I believe it is appropriate at this point to state that I do not condone the drinking of alcohol whilst operating a faceting machine.
You would not believe the fun and games I have had with this little sucker…
Final result: 0.5ct – so yield about 25%, which I am reasonably happy with given the initial shape of the rough.
Aim is to take some good photos tomorrow, then pack it all up and ship it out!