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 Post subject: Sapphire specific gravity
PostPosted: Thu Jul 01, 2021 5:15 am 
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Is it possible for untreated sapphire to have specific gravity as low as 3.80 gr/cc ?


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 Post subject: Re: Sapphire specific gravity
PostPosted: Thu Jul 01, 2021 7:18 am 
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That is two low. But properly measuring SG requires very strict methodology and very precise equipment. Your scale must be accurate to 3 decimal places. The smaller the stone the more likely it is that the measurements are off.

What size is your stone?


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 Post subject: Re: Sapphire specific gravity
PostPosted: Thu Jul 01, 2021 8:58 am 
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According to the Gem-A, and what I learned for the diploma, corundum / sapphire can be between 3.8 - 4.05. It all depends on the quality, fissures and matrix, if still attached. And last but not least measurement errors (related to the size of the stone) and precision of the balance. Personally I wouldn't dismiss or accept a stone on SG measuremts alone when they are close to the "official" figure.


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 Post subject: Re: Sapphire specific gravity
PostPosted: Thu Jul 01, 2021 1:29 pm 
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I agree, when I was learning some of the basic testing of gemstones, I was using single and double digit scales and mostly on the cheap side of the scale market. The readings were always so lets say rounded off and when checking the reference material many times the results were so close it was hard to ascertain where to place the result. So it was time to learn to do the other basic tests for further identification, like RI, SR/DR and color of the gem helps significantly. A scale with a readout in carats to three digits is a big plus.
What is this designation (gr/cc)?

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 Post subject: Re: Sapphire specific gravity
PostPosted: Thu Jul 01, 2021 11:17 pm 
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glhays wrote:
What is this designation (gr/cc)?


grams per cubic centimeter


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 Post subject: Re: Sapphire specific gravity
PostPosted: Fri Jul 02, 2021 1:03 pm 
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Thank you Brian.

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 Post subject: Re: Sapphire specific gravity
PostPosted: Fri Jul 02, 2021 2:09 pm 
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HenkRijneveld wrote:
Personally I wouldn't dismiss or accept a stone on SG measuremts alone when they are close to the "official" figure.

One test is rarely enough to positively ID something.


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 Post subject: Re: Sapphire specific gravity
PostPosted: Fri Jul 02, 2021 6:11 pm 
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My GIA Refence manual shows corundum @4.00 +.10/-.05, which would make it 4.10 or 3.95 would it not?

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 Post subject: Re: Sapphire specific gravity
PostPosted: Fri Jul 02, 2021 7:39 pm 
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HenkRijneveld wrote:
According to the Gem-A, and what I learned for the diploma, corundum / sapphire can be between 3.8 - 4.05. It all depends on the quality, fissures and matrix, if still attached. And last but not least measurement errors (related to the size of the stone) and precision of the balance. Personally I wouldn't dismiss or accept a stone on SG measuremts alone when they are close to the "official" figure.



Exactly.

That is why I brought up the issues of resolution of the scale, methodology, stone size, and form factor.

Even if a result is slightly outside the correct range for a material, when the margin of error is large enough to overlap, then it is not a reliable test.


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 Post subject: Re: Sapphire specific gravity
PostPosted: Sat Jul 03, 2021 4:39 pm 
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it is somewhat confusing. Gem-A gives a S.G. of 3.80-4.05, Webster 3.9-4.0, berkeley.edu 3.96-4.01, mindat 3.98-4.01, gemdat 3.97 to 4.05, GIA (ruby) 3.95-4.05 and our friend wikipedia thinks 3.95-4.10 for corundum is the correct answer.

You wonder were these numbers are coming from. It seems a lot of publishers should recalibrate their scales.

I suppose the main origin of these differences is that one source uses the pure stuff, while the other (like Gem-A) thinks it is acceptable to give a value with fissures, (glass) fillings etc.


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 Post subject: Re: Sapphire specific gravity
PostPosted: Sat Jul 03, 2021 7:25 pm 
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May I presume you have no other instruments to verify this unknown with?
Refractometer? Scope?

If not, may I suggest that you test the SG of a gem you have already positively identified. Like an amethyst.
Should be a slam dunk right around 2.65.
Once you verify that your methods are solid (or not), you can trust your reading of 3.8 is accurate (or not).

When taking the SG on corundum, I like the sound of 4.

Report back.


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