A pyramidion is the uppermost piece or capstone of an Egyptian pyramid in archaeological parlance. In Egypt's Old Kingdom, the pyramidion was generally made of diorite, granite, or fine limestone, which was then covered in gold or electrum, while during the Middle Kingdom and through to the end of the pyramid building era, they were built from granite. A pyramidion was, "covered in gold leaf to reflect the rays of the sun." During Egypt's Middle Kingdom, they were often, "inscribed with royal titles and religious symbols."

Source: wikipedia.org