A touchstone is a small tablet of dark stone such as fieldstone or slate, used for probing of precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace. Jump to: navigation, search Slate Slate is a fine-grained, homogeneous, sedimentary rock composed of clay or volcanic ash which has been metamorphosed (foliated) in layers (bedded deposits). ... A precious metal is a rare metallic element of high, durable economic value. ... An alloy is a combination, either in solution or compound, of two or more elements, which has a combination of at least two metals, and where the resultant material has metallic properties. ... Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) in a cloud of electrons. ...

By rubbing the metal on a touchstone a small amount of metal would be ground off onto the stone, and form a colored stripe. This is typically compared to a stripe ground from an alloy of known high-quality composition next to the sample (this is called "priming" the stone).

The simplest, but crude, method of probing is comparing the color of rubbed stripes, which varies depending on the amount of the base metal in the alloy. However, the standard probing method involves subjecting the stripes to graded solutions of acid which dissolve the alloys with lower content of precious metal but don't affect the alloys with higher content of it. Modern touchstone kits include the touchstone tablet proper, flasks of acids graded for standard alloys — e.g. 18K (karat) (75%) gold, 14K (58%), etc. — and priming pencils made from the standard alloys. Jump to: navigation, search An acid (often represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. ... Carat or Karat may refer to: a unit of mass for gems. ...

Use of the touchstone revolutionized the concept of money. Prior to its introduction gold and silver were common currencies, but these could easily be alloyed with a less expensive metal (tin and lead were common). These were less valuable, but it was difficult to test for. The invention of touchstone made it possible to test for such forgeries quickly and efficiently, and also to determine the relative value of different alloys. That paved the road for gold and silver to become standard equivalents of value, and eventually to government-issued currency which began as coins of pre-probed alloys and weights guaranteed by the mint. Money Money is any marketable good or token used by a society as a store of value, a medium of exchange, and a unit of account. ... Jump to: navigation, search General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ... Jump to: navigation, search General Name, Symbol, Number silver, Ag, 47 Chemical series transition metals Group, Period, Block 11, 5, d Appearance lustrous white metal Atomic mass 107. ... Jump to: navigation, search General Name, Symbol, Number tin, Sn, 50 Chemical series poor metals Group, Period, Block 14, 5, p Appearance silvery lustrous gray Atomic mass 118. ... Lead - Wikipedia /**/ @import /skins-1. ... Species See text The true mints are perennial herbs in the family Lamiaceae. ...