Arkansas stone

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Arkansas stone is a, polycrystalline, silica (chalcedony) rich chert that is often used as a whetstone. Bench jewelers often use this stone as a sharpening tool for gravers. Very fine grained stones are used to give a high polish to the sharp edges of tool, making them even sharper.
Appraisers use the black, opaque variety as a touch stone to determine the quality of precious metals (in conjunction with acids).
The mineralogical name of this rock is novaculite.

The specific gravity of the material varies with grit size and ranges from approx. 2.2 to 2.5.
Novaculite is found in Arkansas, Texas and Oklahoma (all USA).