Rare elements are lost secrets, ancient magic, and other options that PCs can access only if you specifically make them available. If you know how to homebrew items, and how the Inventor uses the Inventor feat to craft the formula for a common item, then all you're missing is an understanding of rarity:

Johannes Hartlieb (1410-1468) wrote a compendium on herbs in ca. 1440, and in 1456 the puch aller verpoten kunst, ungelaubens und der zaubrey (book on all forbidden arts, superstition and sorcery) on the artes magicae, containing the oldest known description of witches' flying ointment.

Agrippa himself was famous as a scholar, physician jurist, and astrologer, but throughout his life he was continually persecuted as a heretic. His problems stemmed not only from his reputation as a conjurer, but also from his vehement criticism of the vices of the ruling classes and of the most respected intellectual and religious authorities. [ This quote needs a citation]

Johannes Trithemius (1462–1516) Trithemius' most famous work, Steganographia (written c.1499; published Frankfurt, 1606), was placed on the Index Librorum Prohibitorum in 1609 [143] and removed in 1900. [144] This book is in three volumes, and appears to be about magic—specifically, about using spirits to communicate over long distances. However, since the publication of a decryption key to the first two volumes in 1606, they have been known to be actually concerned with cryptography and steganography. Until recently, the third volume was widely still believed to be solely about magic, but the magical formulae have now been shown to be covertexts for yet more material on cryptography. [145] [146] In my academic pursuits, I earned a BSc in Information Systems Design from UCLAN, before advancing my learning with a Master’s Degree in Computing. This advanced study also included Cisco CCNA accreditation, further demonstrating my commitment to understanding and staying ahead of the technology curve.As educated elites in Western societies increasingly rejected the efficacy of magical practices, legal systems ceased to threaten practitioners of magical activities with punishment for the crimes of diabolism and witchcraft, and instead threatened them with the accusation that they were defrauding people through promising to provide things which they could not. [202] The practice of magic was banned in the late Roman world, and the Codex Theodosianus (438 AD) states: [76] Johann Georg Faust (c. 1480 or 1466 – c. 1541) was a German itinerant alchemist, astrologer and magician of the German Renaissance. Because of his early treatment as a figure in legend and literature, it is difficult to establish historical facts about his life with any certainty. It was always Pico's aim to reconcile the schools of Plato and Aristotle since he believed they used different words to express the same concepts. It was perhaps for this reason his friends called him "Princeps Concordiae", or "Prince of Harmony" (a pun on Prince of Concordia, one of his family's holdings). [140] Similarly, Pico believed that an educated person should also study Hebrew and Talmudic sources, and the Hermetics, because he thought they represented the same concept of God that is seen in the Old Testament, but in different words.