In this chapter I show how pyrite was at the heart of our early understanding of the composition of substances and how it was central to the acceptance of the revolutionary idea that substances have fixed compositions. This, in turn, was the evidential basis for the modern atomic theory. Taxonomists will argue that naming things accurately is important since otherwise no-one will know what you are talking about. They would disagree with Shakespeare that a rose would smell as sweet whatever it was called on the grounds that you would not know that a rose was being described. Even so, the only reason things are named is because of need. Thus Homer did not have a word for blue because he never needed it: the blue sea became wine-dark, for example. By contrast, contemporary ancient Egyptians had a word for blue because they used the blue mineral lapis lazuli for decoration. The mineral pyrite has been employed by humankind for millennia, and it needed a name. Its long history means that a variety of terms have been used to describe it, often reflecting the technology available at the time. In order to understand the role that pyrite has played in the past, we need to interpret the various names given to this mineral by earlier authorities. This problem is compounded since its history is determined by ancient texts and these were commonly written down by scribes from direct dictation. The scribes rendered the sounds of words as best they could within the limitations of the current orthography. Before the advent of printing, copyists made reproductions of these original texts according to the customs and mores of their local culture. The texts that have come down to us are usually the result of the work of several generations of copyists, and the interpretations become like a game of Chinese whispers. Whether or not a word in an ancient text means pyrite is, at best, a matter of relating it to a description that reflects key properties of the mineral. At worst it may mean probing the etymology of the word and considering its context.