The word Touchstone has to me always conjured up something magical possibly having the mystical property of turning worthless metals into gold. But if we turn this idea around and think of it as a way that will tell us if something is gold and not a worthless imitation then we are approaching its true function.

Touchstones are in fact a way of assaying the purity of gold alloys and the method simply involves making a streak of the metal to be tested on a Touchstone and comparing that streak after treatment with nitric acid with a series of standards rods.

The geology angle comes in when one asks the question ; what is a touchstone made of? The following extract from Cripps (1891) explains the nature and use of Touchstones.:

" But as these early times do not immediately concern the present inquiry, we must pass on to the mode used in what are called the Middle Ages, and even more in more modern times, of testing the fineness of gold and silver by the touchstone, or pierre de touche. King Henry VII by his will directs that 'there be made a tomb of stone called Touche sufficiently large both for our dearest late wife of the Queen and ourself. This Touchstone or Basanite is an imperfect black jasper or black flinty slate, originally brought from Mount Timolus in Lydia, and therefore called lapis Lidius; it is however found in various parts of the world, and indeed any hard black siliceous substance , or even a piece of black pottery, will serve the purpose. The great Josiah Wedgwood made such, stamped WEDGWOOD / ETRURIA about 1770 or 1780.
The mode of trying the fineness was called "touching" and the word was obtained for a long time after the adoption of the chemical assay. The word "touch" seems to have been applied indifferently to the trial, to quality of the metal tested, and to the mark impressed upon it. ...

...... For the trial of gold, sets of touch-needles or bars were used, one set alloyed with copper, another with silver, and in some cases a third set alloyed with silver and copper mixed twenty four in each set according to the twenty-four carats' fineness of gold. The streak or touch made on the touch stone with the piece under examination was compared with the streaks made by the needles, these streaks were also washed with aquafortis (nitric acid), which dissolving alloyed metals, left behind the pure gold, and by comparison its fineness was determined."

In our own way the principle of touch stones has continued in geology in as much as mineral identification can be assisted by making a streak on an un-glazed white tile, its colour being a guide to its composition. These are given in detail in Rutley's Mineralogy (1962) and although low-tech it is a useful technique to use in the field and should not be ignored.

References

Cripps W.J. (1891). Old English Plate , p 11, published by John Murray
Albemarle St., London.

Rutley (1962) Elements of Mineralogy, 25th edition. Ed by H.H. Read, published by Thomas Murray & Co, London.

(Essex Rock & Mineral Society (member's article July 2002)

(Other articles by members)