FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hi,
Here is a gemologist tool that seems rare.
A crown carries 6 stones to test Mohs hardness.
The values are: 6.3/4 7 7.1/2 8 8.1/2 9
What are these stones?
What is the name of the tool?
Roger.
PS: It’s seems to me that 9 is a sapphire and 7.1/2 a garnet.

[gemlover]

We used wheels like this in my mineralogy course in college (50 years ago). I have not seen one since then. Thanks for the memories. BTW, a gemologist would not use this, as we don't scratch the cut gems.

However a mineralogist could.

John

[vicen]

Quite an interesting tool. Is it still in the market? Tá

[Roger Warin]

Oh no! This wheel was in an old collection.
Its profile is similar to that of a trapiche wheel used in crushing rocks.
Roger.

[Mark Ost]

Quite remarkable. Interesting that the gradations go to fractional units.

[Riccardo Modanesi]

Hi to everybody!
Do you think this instrument is safe to be used on cut stones? Some 30 or 40 years ago there was an instrument called sclerometer, which was a pool of "points" made of the 10 Mohs scale terms (talc, gypsum, chalcite etc.). It wasw abandoned because it was said too dangerous for cut stones; in particular the diamond point is dangerous even for a cut diamond itself! If it is useful for rough stones (i.e. crystals), it's ok, but I doubt about its usefulness on cut stones!
Greetings from Italy by Riccardo.

[Roger Warin]

Hello,
It is likely that this is a hand held instrument for a geologist on the ground. It could of course be used before cutting of a gemstone. Currently, this is an obsolete antique.
Roger.

[vic rzonca]

Roger, what would be the modern replacement for this tool? Perhaps the go-no go hardness of a Mohs 5 implement is all that's needed? It is an elegant object.

Vic

[Mark Ost]

A guess metal point sets are the new analog of this tool. I have a boxed set that is graduated on the whole number mohs scale. No fractional reading.

[Mark Ost]

Here is what I use

[vic rzonca]

Great Mark, thanks. Gonna git me a set.

[cascaillou]

I wouldn't scratch a cut stone. Actually, it's best to avoid any potentially destructive testing (i.e. hardness, streak, acid) when dealing with cut stones. On the other hand, if all classical gemology non-destructive tests have already been implemented, and I'm left with only two or three possibilities for the mineral identity, and if those could be easily separated by their substantial hardness difference (like 2 or more Mohs graduations), then I might consider it. Even then, I wouldn't scratch the cut stone, but rather scratch a flat smooth polished plate of known hardness with the gemstone (do not use rough unpolished pieces of the mohs scale minerals).
But that's risky too as the stone might chip if its hardness is equal or inferior to the polished plate. Therefore one should start with a low hardness plate (meaning lower than the hardness of your softer candidate for the stone identity) and work up progressively, applying little pressure during scratching. As the plates get harder, scratching these with the stone gets less easy, and if at some point you can't get a scratch then do not try applying more pressure and just stop the hardness testing at this point. Anyway, at that point you already have an approximative idea of what the hardness is anyway (let's consider you might have an error of 1.5 Mohs graduation in the worth case) , which might be sufficient to orientate your choice between the 2 or 3 possibilities left for your mineral identity.

Of course always make sure that the scratch on the test plate is actually a scratch, not a streak, you can make sure by:
rubbing it vigorously with your moist finger tip (unlike a streak, a scratch shouldn't disappear when doing so), and then scratch it perpendicularly with your finger nail (a scratch is a groove so it should catch your nail a bit)

That's why I think hardness points aren't a good choice (as you'll likely end up scratching your stone), I'd rather use polished hardness plates (it's also risky, but if all goes well you can get an approximative idea of the stone hardness without damaging it).

However, if you want to test uncut minerals, then it's best to have both points and plates (to get more accurate hardness result, meaning 0.5 mohs graduation accuracy, you need to know both what the stone can scratch and what the stone can't be scratched with)