Specific Gravity test help - Gold and quartz

for what it's worth.......If it's ugly...in the dolly pot it goes. If it's 'purty' and if I was going to sell it (which I don;t) then I would be selling it for spot price as it weighs.
So...if it weighs 10g then that's what I would charge for...as long as whoever buys it knows that they are not buying 10g worth of gold. Thing is, anyone buying native gold nuggets are never getting what they pay for as far as gold content goes.

Thanks for your replies guys. Is there an app that can be downloaded, where you just put in your dry weight,then your wet weight, and it calculates the gold content? That would be a lot simpler.

Just a reminder of how to do an SG Test.

Goldtalk Leonora said:

where your going wrong is using a SG test at all....whatever figure you come up with is unreliable.
The only way of knowing the gold content is to put it in the dolly pot.

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If it is in pure quartz you can estimate it very accurately. You can find the combined SG and you know the SG of gold and quartz. So you can calculate how much gold has to be added to quartz to give the combined SG. You can get a fair estimate even if it is not in quartz, by using an approximate rock SG if you know the rock. However, you will be fairly close if you use a value like quartz (2.65 - corrected since first post) since rocks only range from 2.2 to 3.4 and gold is 19.3, A huge difference. Only problem is when there are a lot of other sulphide minerals. This is how specimens are valued. You shoukld not lose with a good specimen because you should be charging a surcharge over the actual gold value.

I have never been good at remembering things like formulae which many use to calculate gold content from wet and dry spec weights and use the following estimation process.

The example at the start of this topic assumed a dry weight of 25.68 gr and I assume that the author meant a weight loss of 2.99 gr when weighed suspended in water. To estimate the gold content, I use the following method.

A 2.99 gram loss in weight would equate (Archimedes principle) to a volume of 2.99 cc for the specimen which if all quartz would weigh just 2.99 x SG (2.62) = 7.83 gr. That would mean that the rest of the weight must be the gold content ie. 25.68 -7.83 = 17.85 gr of gold.
This is of course a minimum first estimation and can be refined further as we know that the gold itself would occupy some of the specimen volume we attributed to quartz in this first estimation.

For a second estimation, we can then calculate that 17.85g of gold would occupy a volume of 17.85 divided by its SG, 17.85 divided by 19.3 = 0.92 cc. This reduces the amount of quartz volume to 2.99 – 0.92 = 2.07 cc. Then again this volume of quartz would only weigh 2.07 x 2.62 = 5.42gr. Therefore our estimation of gold should be increased to 25.68 – 5.42 = 20.26gr of gold.

A third iteration of this process can be done to improve the gold estimation to 20.60gr and yet a fourth improves the estimation to 20.65gr.

No point in going any further due to the inherent weighing, specimen mineral and SG variations.

goldierocks said:

If it is in pure quartz you can estimate it very accurately. You can find the combined SG and you know the SG of gold and quartz. So you can calculate how much gold has to be added to quartz to give the combined SG. You can get a fair estimate even if it is not in quartz, by using an approximate rock SG if you know the rock. However, you will be fairly close if you use a value like quartz (2.95) since rocks only range from 2.2 to 3.4 and gold is 19.3, A huge difference. Only problem is when there are a lot of other sulphide minerals. This is how specimens are valued. You shoukld not lose with a good specimen because you should be charging a surcharge over the actual gold value.

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Sure, but I have rarely seen a specimen in 'pure QTZ". Nuggets and species always contain impurites....that is why we loose around 7-15% in the furnace. If you want to know the gold content then the only reliable...repeatable method is to bash it up...in my opinion. Value of a specie is a different kettle of fish....on the rare occasion I sell a specie, I sell it by the total weight at spot price. So, if the specie weighs an ounce....then I charge an ounce. As long as the buyer understands the are not paying for an ounce of gold then everyone is happy.

Of course all the methods used to calculate gold content use assumed SG values for quartz around 2.60 -2.65 depending on what reference you use, and even the assumed of 19.3 for gold can be different depending on the alloy content.
As Goldtalk Leonora and goldierocks suggest quartz is often not pure, although many of the specis I've found in Victoria are often in pure white quartz or close to that. I have also found many specis in highly coloured quartz and in other mineralised quartzy matrices.
If you have any doubts that the quartz in your speci is not close to its standard SG, just look around for a similar looking piece of quartz (without gold) laying nearby as a reference piece and do a simple SG measurement on it while you have your gear set up.
The SG for quartz you would use in the formula is just the reference piece's dry weight divided by it's wet weight.
I know some have said "who cares about calculating just smash it and weigh the gold" or "just sell it as a speci for a premium price" but some of us do really want to know.
The wife and I have kept a couple of nice specis as prized pieces for show and tell and the first question we get asked is "How much gold is there in it?.
We do not like to be put in a position of having to guess.

Hawkear said:

Of course all the methods used to calculate gold content use assumed SG values for quartz around 2.60 -2.65 depending on what reference you use, and even the assumed of 19.3 for gold can be different depending on the alloy content.
As Goldtalk Leonora and goldierocks suggest quartz is often not pure, although many of the specis I've found in Victoria are often in pure white quartz or close to that. I have also found many specis in highly coloured quartz and in other mineralised quartzy matrices.
If you have any doubts that the quartz in your speci is not close to its standard SG, just look around for a similar looking piece of quartz (without gold) laying nearby as a reference piece and do a simple SG measurement on it while you have your gear set up.
The SG for quartz you would use in the formula is just the reference piece's dry weight divided by it's wet weight.
I know some have said "who cares about calculating just smash it and weigh the gold" or "just sell it as a speci for a premium price" but some of us do really want to know.
The wife and I have kept a couple of nice specis as prized pieces for show and tell and the first question we get asked is "How much gold is there in it?.
We do not like to be put in a position of having to guess.

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Hi Geoff.....and don't forget that it's good fun trying to work the gold content!! It's an interesting thing to do with one of natures gifts....all I am saying is I would not base a sale/purchase price on it as there are far too many variables and it is an unreliable method. Of , short of bashing it up, it's the best method we have.

Hi GoldTalk, No problem here, I have probably crushed as many specis as most. At the end of the day most of us would like to cash in as best we can and for whatever reason.
I think most of us would describe our activity more in terms of the thrill of the hunt and finding of rather than in the possession of gold.
Doing the SG test is just another element of that thrill. The result can be disappointing or gratifying.
Just to add some further illustrations as to how the SG approximation method works, I've done the following graph of how accuracy is achieved over a number of simple approximations without the need to remember complicated formulae. As can be seen after about three approximation starting from an assumption of all quartz and no gold a very accurate (good enough) approximation of gold content can be achieved in this case 20.65gr. (Dry weight 25.68g wet 22.69g)
Edit - I should have titled graph "Gold Content Progressive approximation method"

Goldtalk Leonora said:

Sure, but I have rarely seen a specimen in 'pure QTZ". Nuggets and species always contain impurites....that is why we loose around 7-15% in the furnace. If you want to know the gold content then the only reliable...repeatable method is to bash it up...in my opinion. Value of a specie is a different kettle of fish....on the rare occasion I sell a specie, I sell it by the total weight at spot price. So, if the specie weighs an ounce....then I charge an ounce. As long as the buyer understands the are not paying for an ounce of gold then everyone is happy.

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The SG method means you don't destroy the sample. Unless there are sulphides most rocks are similar SG to quartz (and you can see the rock type so get an ever better estimate). And with a "museum" type specimen there will be a significant surcharge above gold value. Gold can have as much as 50% silver but in most places it is less than 10%, and near surface almost always. Typical Victorian gold is above 94% even at depth, and we know where it is otherwise in Victoria (St Arnaud, Omeo).

Have used this to value samples for Sovereign Hill museum for purchase (eg 40 oz contained gold in fairly pure quartz)

Hawkear said:

Of course all the methods used to calculate gold content use assumed SG values for quartz around 2.60 -2.65 depending on what reference you use, and even the assumed of 19.3 for gold can be different depending on the alloy content.
As Goldtalk Leonora and goldierocks suggest quartz is often not pure, although many of the specis I've found in Victoria are often in pure white quartz or close to that. I have also found many specis in highly coloured quartz and in other mineralised quartzy matrices.
If you have any doubts that the quartz in your speci is not close to its standard SG, just look around for a similar looking piece of quartz (without gold) laying nearby as a reference piece and do a simple SG measurement on it while you have your gear set up.
The SG for quartz you would use in the formula is just the reference piece's dry weight divided by it's wet weight.
I know some have said "who cares about calculating just smash it and weigh the gold" or "just sell it as a speci for a premium price" but some of us do really want to know.
The wife and I have kept a couple of nice specis as prized pieces for show and tell and the first question we get asked is "How much gold is there in it?.
We do not like to be put in a position of having to guess.

Click to expand...

Yes 2.65 (2,95 a typo on my part, that is a bit high).

There was a Excel gold calculator floating around years ago that I think was put together by a fella named Bill & wife Sue? Bill was on the admin side of a gold forum? The calculator was very handy but somehow I inadvertantly deleted it. All I have is a photo of what it looked like.
We have never crushed any of our specimens, they look far better than a blob of gold. Can anyone help?

There are some formulae around that purport to calculate gold content but these are generally arithmetic formulae and estimatory and often translated into plug in applications. The true relative content can only be obtained by calculus methods.
Calculus is based on common sense idea of doing progressive estimations to approach a true value limit.
All you have to do is assume that the entire specimen volume (easily obtained by the water uplift method) is composed of quartz. If so what weight should that volume of quartz weigh? That is easily obtained by multiplying the volume by the SG of quartz. Obviously any weight more than that should be gold.
Then ask yourself, if not all the volume was quartz what volume does that estimated weight of gold occupy? That is also easy to calculate by dividing that weight by gold’s SG and that volume then has to be subtracted from the volume we just assumed to be fully occupied by quartz.
When this newer estimation of quartz volume is made a revised and more accurate estimation of gold weight can be made.
This process can be repeated as many times as wanted to give an ever increasing accurate gold content figure. After about 4 iterations however no significant change in the gold content will be noted as this is the calculus limit.
In practice only about 3 iterations are necessary as natural variations in gold and quartz SGs may be present as well as measurement errors.

Hawkear said:

There are some formulae around that purport to calculate gold content but these are generally arithmetic formulae and estimatory and often translated into plug in applications. The true relative content can only be obtained by calculus methods.
Calculus is based on common sense idea of doing progressive estimations to approach a true value limit.
All you have to do is assume that the entire specimen volume (easily obtained by the water uplift method) is composed of quartz. If so what weight should that volume of quartz weigh? That is easily obtained by multiplying the volume by the SG of quartz. Obviously any weight more than that should be gold.
Then ask yourself, if not all the volume was quartz what volume does that estimated weight of gold occupy? That is also easy to calculate by dividing that weight by gold’s SG and that volume then has to be subtracted from the volume we just assumed to be fully occupied by quartz.
When this newer estimation of quartz volume is made a revised and more accurate estimation of gold weight can be made.
This process can be repeated as many times as wanted to give an ever increasing accurate gold content figure. After about 4 iterations however no significant change in the gold content will be noted as this is the calculus limit.
In practice only about 3 iterations are necessary as natural variations in gold and quartz SGs may be present as well as measurement errors

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I wish that you were my high school maths teacher. The question of "Why do we need to know this?" is answered right here.

Campfish4x4 said:

Hi all I've gotta specie that weighs 25 .68 g dry and 2.99g underwater on a line so how much qaurtz is hedden in the nugget thanks and merry Xmas to all

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if your weights are accurate then minus the 2.99 from the 25.68 = 22.69, so aprox 20.74 grams of gold, if what you say it is mostly gold this is the way to do it, I use this spreadsheet calculator and just input the weights

I wanted to post an Excel specific gravity calculator but I can't work out how to do it? (*.xls)

Moneybox said:

I wanted to post an Excel specific gravity calculator but I can't work out how to do it? (*.xls)

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Doesn't appear to be possible, Phil - I just tried.

This ".xls" will allow calculation with different sg for host materials when you download it. When you open the spreadsheet just use the save feature on your down-loaded file.
Link...> Spreadsheet 4 Calc of Gold.