SG Calculation

I do see a lot of people doing SG’s the hard way using all sorts of jigs and contraptions to try and measure the weight of the stone in water. There’s a simpler way – you just need to measure the weight of the water displaced by the stone when fully immersed.

You will only need an accurate carat scale (accuracy to at least .01g), a small beaker/glass of water, a calculator and some fine cotton.

Weigh the piece of aquamarine: 2.06g
Tie some cotton to the stone, use a slip-knot, or make a little cotton sling for small cut stones.
Zero a beaker of water on the scales
Measure the weight of the water displaced by totally immersing the stone supported by cotton, just below the surface, not touching anything: 0.76g

Divide 2.06 by 0.76

Answer: 2.71

That’s it; done.

Walter Schumann Gemstones of the World says for aqua SG: 2.68 – 2.74

Bang in the middle!


I recently went on a short prospecting trip to the south-west of Tasmania with a couple of friends. The main aim was to locate some streams that might hold a bit of alluvial gold. We ran into a lot of problems with road access – forestry roads that were closed due to the bushfires from last summer or for other reasons, so did not really achieve our aims.

On one small track that ended in a small disused quarry, a shattered old TV set lay on the ground, dumped years ago by some vandalous low-life.


This was one of those old bulky TV sets with a glass CRT Cathode Ray Tube (yes, you heard right youngsters – no flat-screen in sight here). Flat screen TV’s apparently out-sold CRT TV’s from 2008 onwards to give you some point of reference. The glass TV tube was now simply a collection of angular glass fragments of different sizes and shapes scattered amongst the crushed TV shell and around in the nearby vegetation. No doubt the unknown person(s) who went to the trouble of transporting it all the way out here in the wilderness had also taken perverse pleasure in enthusiastically imploding the cathode ray tube with whatever lay at hand. So had it been a family outing? C’mon kids, stop watching that new Samsung, we’re going for a drive in the country, bring a warm jacket and your favourite rock.

On discovering the technological remains of times past, I began to think of the irony as I bent down to collect some pieces. As it happens, the glass used in the manufacture of CRT’s was very good quality glass of specific properties. I know this because I have faceted CRT glass before, and it is remarkably clear optically – high clarity with very few visible inclusions. The colour of this glass varied from shades of grey, to bluish-grey to yellowish brown, however I suspect the yellowish colour may be a weathering effect.

If we consider synthetic rough versus simulant rough for a moment:

In contrast to synthetic gemstones which have the same chemical composition, crystal system and physical constants as their natural counterparts, a simulant need only have a superficial resemblance to the gemstone it imitates.

Gemmology (Peter G. Read, 1991)

So, on this day instead of finding alluvial gold, I had found what in effect was gemstone rough – or at least a simulant “LG-ite” – the LG branding still visible on the shattered TV case.

I have also sourced CRT glass in the past from beaches near Hobart, deposited there as the result of nefarious activities – Rank Arenarite? The further irony being that CRT tubes are no longer manufactured, so this glass will become rarer and rarer as time goes on – much like natural gem material.

As an addendum, it must be cautioned that CRT glass can/does have a significant lead oxide (PbO) component. So it is probably not a good idea to work with this material for extended periods, or long term, due to the potential health risks of lead.