touch-needle

from The Century Dictionary.

• noun One of a series of strips or needles of various alloys of gold, silver, and copper of known composition, used in testing the quality of gold by the use of the touchstone.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Metal.) A small bar of gold and silver, either pure, or alloyed in some known proportion with copper, for trying the purity of articles of gold or silver by comparison of the streaks made by the article and the bar on a touchstone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small bar of gold and silver, either pure, or alloyed in some known proportion with copper, for testing the purity of gold or silver articles by comparison of the streaks made by the article and the bar on a touchstone.