[Copied below a couple of posts I made on B&B recently. It's probably talked about more there than it is here, but here's a run down anyway...]
This is something I find myself explaining or pointing out to people quite a lot, and I'm going to write it out in case it's helpful for anyone, and so that next time I can send a link.
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Specific gravity is a measure of density. It is not a measure of hardness, and it is not a measure of fineness.
The confusion arises because for the stones that it's most often talked about in relation to - Arkansas novaculites - specific gravity does correlate with both both hardness and fineness. However this is not necessarily true for other types of stone, and you cannot compare specific gravities across types of stone and try to draw meaningful conclusions.
A slate might have an SG around 2.80. Is it a harder and finer stone than a translucent Ark at 2.64? No. A coticule might come in at 3.00. Ditto. You can't even use it to compare two different novaculites; a Charnley has an SG around 2.70, and is again is a softer and coarser stone than a trans or black Ark. To look at the specific gravity of something and try to infer characteristics from it, you need to know about both the chemical composition and structure of what you're looking at.
And even when comparing stones of the same type SG does not necessarily have a +ve correlation with fineness. For Idwals and Charnleys; the finer stones tend to have lower specific gravities, closer to that of silica at 2.65. They are purer novaculites than examples with higher SGs. Looking at slates, Thuringians, &c. SG is all over the place, it doesn't correlate positively or negatively with the fineness of the grit (ime), or the hardness of the stone, seemingly random. It's a product of the composition of initial deposits, and how metamorphic change has affected them, and that's a very complicated equation indeed.
SG is however a useful measurement when trying to determine the identity of an unknown stone. If you have a very fine green-grey hone with an SG of 2.55 - it is not a Thuri, but it could be an Asagi &c. &c.
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That's all really. It's an easy trap to fall into given that traditionally SG is (legitimately) used to look at the fineness of Arkansas stones. But be clear that this equivalence does not always hold for other things.
This is something I find myself explaining or pointing out to people quite a lot, and I'm going to write it out in case it's helpful for anyone, and so that next time I can send a link.
---
Specific gravity is a measure of density. It is not a measure of hardness, and it is not a measure of fineness.
The confusion arises because for the stones that it's most often talked about in relation to - Arkansas novaculites - specific gravity does correlate with both both hardness and fineness. However this is not necessarily true for other types of stone, and you cannot compare specific gravities across types of stone and try to draw meaningful conclusions.
A slate might have an SG around 2.80. Is it a harder and finer stone than a translucent Ark at 2.64? No. A coticule might come in at 3.00. Ditto. You can't even use it to compare two different novaculites; a Charnley has an SG around 2.70, and is again is a softer and coarser stone than a trans or black Ark. To look at the specific gravity of something and try to infer characteristics from it, you need to know about both the chemical composition and structure of what you're looking at.
And even when comparing stones of the same type SG does not necessarily have a +ve correlation with fineness. For Idwals and Charnleys; the finer stones tend to have lower specific gravities, closer to that of silica at 2.65. They are purer novaculites than examples with higher SGs. Looking at slates, Thuringians, &c. SG is all over the place, it doesn't correlate positively or negatively with the fineness of the grit (ime), or the hardness of the stone, seemingly random. It's a product of the composition of initial deposits, and how metamorphic change has affected them, and that's a very complicated equation indeed.
SG is however a useful measurement when trying to determine the identity of an unknown stone. If you have a very fine green-grey hone with an SG of 2.55 - it is not a Thuri, but it could be an Asagi &c. &c.
---
That's all really. It's an easy trap to fall into given that traditionally SG is (legitimately) used to look at the fineness of Arkansas stones. But be clear that this equivalence does not always hold for other things.
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Not really... it is "yet another scale"....
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