existence in South India before the Christian era. Evidence for using bow-instruments for carding comes from India (2nd century CE). The mining of diamonds and its early use as gemstones originated in India. Golconda served as an important early center for diamond mining and processing. Diamonds were then exported to other parts of the world. Early reference to diamonds comes from Sanskrit texts. The Arthashastra also mentions diamond trade in the region. The Iron pillar of Delhi was erected at the times of Chandragupta II Vikramaditya (375–413), which stood without rusting for around 2 millennium. The Rasaratna Samuchaya (800) explains the existence of two types of ores for zinc metal, one of which is ideal for metal extraction while the other is used for medicinal purpose. The origins of the spinning wheel are unclear but South Asia is one of the probable places of its origin. The device certainly reached Europe from India by the 14th century. The cotton gin was invented in South Asia as a mechanical device known as charkhi, the "wooden-worm-worked roller". This mechanical device was, in some parts of the region, driven by water power. The Ajanta Caves yield evidence of a single roller cotton gin in use by the 5th century. This cotton gin was used until further innovations were made in form of foot powered gins. Chinese documents confirm at least two missions to India, initiated in 647, for obtaining technology for sugar-refining. Each mission returned with different results on refining sugar. Pingala (300–200 BCE) was a musical theorist who authored a Sanskrit treatise on prosody. There is evidence that in his work on the enumeration of syllabic combinations, Pingala stumbled upon both the Pascal triangle and Binomial coefficients, although he did not have knowledge of the Binomial theorem itself. A description of binary numbers is also found in the works of Pingala. The Indians also developed the use of the law of signs in multiplication. Negative numbers and the subtrahend had been used in East Asia since the 2nd century BCE, and South Asian mathematicians were aware of negative numbers by the 7th century CE, and their role in mathematical problems of debt was understood. Although the Indians were not the first to use the subtrahend, they were the first to establish the "law of signs" with regards to the multiplication of positive and negative numbers, which did not appear in East Asian