Graphite is a soft allotrope of carbon, noted for being one of few conductive non-metals. This is caused by its structure; 'sheets' of carbon linked by weak bonds, with every one in two having a 'spare' electron. Its sheet-like structure means that it is often mixed with clay to form the 'lead' in pencils. This has been used as a safer alternative to the lead mineral which had been used until this time in the 1600's.
The graphite used in pencils is a natural substance and varies in its characteristics; deposits that have the characteristics that make good pencils are rare. In 1847 a major discovery of high-quality graphite was made in Siberia, and the region became famous. The association of the best pencils with Oriental graphite explains both the traditional yellow color and the name of the Mongol pencil company.