basanite
Noun
Meaning
Lydian stone, or black jasper, a variety of siliceous or flinty slate, of a grayish or bluish black color. It is employed to test the purity of gold, the amount of alloy being indicated by the color left on the stone when rubbed by the metal.
Source: Webster's dictionarySynonyms
standard
test
stone
guide
measure
gauge
criterion
precedent
hallmark
barometer
touchstone
yardstick
bassanite
Lydian stone
lydite