specific gravity

[shaman15771]

Can someone tell me the formula? I have a very pale pink gemstone I want to check.

 

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[hvacker]

Weight in air divided by the weight in air minus the weight in water

 

[shaman15771]

Thanks!

 

[BurntBear]

Quick Test Method for Determining Specific Gravity of Mineral Specimens

 

[shaman15771]

Thanks Luke I printed it and will check this weekend. Will let you know what I get.

 

[hvacker]

Quick Test Method for Determining Specific Gravity of Mineral Specimens

Hey BB, everything I have on SG is the formula "Dry weight divided by dry weight minus wet weight". The math the guy uses in the web sight won't give that answer. His example only divides the dry weight by the wet weight and results in a SG of 2.62 but using the formula I have I'm seeing the result is 1.61. I have cross referenced the formula I use in several books just to be sure I wasn't having a senior moment. Example: The Gemstone Handbook by A.Thomas.
Can anyone add to this confusion?

 

[BurntBear]

That has me confused, lol.

"Dry weight divided by dry weight minus wet weight"

If a dry weight was 3.0 grams and you divided the dry weight from itself, it would be 1. If you subtracted the wet weight from 1, you would be negative.

 

[hvacker]

In truth I don't think the guy's math can work with anything except evenly divided whole numbers like dry 8 wet 4 or 5 &10, 6&12. I'll stick with the other formula. It's the one I find most everywhere.
His misses the fact that the weight has to reference the original dry weight. So the difference of dry minus wet is the result divided into the original dry weight.
If dry was 3 and wet was 1 then 3-1=2 then 3/2=SG= 1.5 His numbers would be dry=3/wet=1 =SG3 =Can't happen.

 

[BurntBear]

His misses the fact that the weight has to reference the original dry weight. So the difference of dry minus wet is the result divided into the original dry weight.

Who is missing this key factor?

 

[hvacker]

Who is missing this key factor?

Whoops! I was meaning the web site you referenced Quick Test Method for Determining Specific Gravity of Mineral Specimens
His formula isn't valid IMHO

 

[BurntBear]

I'm not sure what you missed. He records the dry weight, records the suspended weight and divides the suspended weight from the dry weight. It works 100% on all of my specimens.

 

[hvacker]

I'm not sure what you missed. He records the dry weight, records the suspended weight and divides the suspended weight from the dry weight. It works 100% on all of my specimens.

Sure you get a result but I question his math because the formulas I find published won't agree with his results. OK take his example. Dry weight 17.6 Wet 6.7 He divides the dry by the wet and his answer is 2.62 SG
Using published formulas subtract the dry from the wet and take that result and divide it into the dry weight you get 1.61SG Same weights different result.
This formula is in at least three of my mineral books.
Like said his formula will only give the same result if the numbers are evenly divisible like 4 & 8 6 & 12.

 

[Eu_citzen]

The correct formula is:

D/(D-W)=SG

Dry Weight / (Dry weight - Wet weight)

Proper example of a citrine:

3.725/(3.725-2.330)= 2.67

The slight variation is due to the not so precise scale I used.

 

[shaman15771]

Thank you!

 

[hvacker]

Yes EU, I think the problem is with equating SG with density. Two different animals although when the numbers are whole numbers can result in similar results. In many publications the density is equated with SG is not an important consideration but when the math is done in many weights it's a huge difference.
Especially with the pesky decimal point.
When weighing a mineral first consider if your trying to establish density or specific gravity and what your base line is.
Your help in this is really appreciated.

 

[Eu_citzen]

Perhaps many do over-complicate this. SG is just a unspecific number, i.e. 2.65 for quartz. It stands in no relation to anything else
Density however has a relationship with volume (cubic centimetre in the example). So compare SG with the density for quartz: 2.65 - 2.66 g/cm[SUP]3


[/SUP]