Book 2 Chapter XIII Paragraph 37

Uniform in colour, equal in the colour of test streak to the standard gold, devoid of hollow bulbs, ductile (sthira), very smooth, free from alloys, pleasing when worn as an ornament, not dazzling though glittering, sweet in its uniformity of mass, and pleasing the mind and eyes,---these are the qualities of tapaníya, pure gold.