Pseudo-Geber (or "Latin Pseudo-Geber") refers to a corpus of Latin alchemist writing dated to the late 13th and early 14th centuries, attributed to Geber (Jābir ibn Hayyān), an early Arabic alchemist. The most important work of the corpus is Summa perfectionis magisterii ("The Height of the Perfection of Mastery"), likely written before 1310 and sometimes attributed to Paul of Taranto. The work was influential in the domain of alchemy and metallurgy in late medieval Europe.
The historicity of Jābir ibn Hayyān itself is in question, and most of the numerous Arabic works attributed to him are, themselves, pseudepigraphic. But it is common practice to refer to the earlier, Greek-inspired body of Arabic texts as the Corpus Jabirianum or Jabirian Corpus, and to the later, 13th to 14th century Latin corpus as Pseudo-Geber or Latin Pseudo-Geber, a term introduced by Marcellin Berthelot. The "Pseudo-Geber problem" is the question of a possible relation between the two corpora. This question has long been controversially discussed. It is not mostly thought that at least parts of Latin Pseudo-Geber are based in earlier Arabic authors such as Al-Razi.
The following set of books is called the "Pseudo-Geber Corpus" (or the "Latin Geber Corpus"). The works were first edited in the 16th century, but had been in circulation in manuscript form for roughly 200 years beforehand. The stated author is Geber or Geber Arabs (Geber the Arab), and it is stated in some copies that the translator is Rodogerus Hispalensis (Roger of Hispania). The works attributed to Geber include:
Being the clearest expression of alchemical theory and laboratory directions available until then—in a field where mysticism, secrecy, and obscurity were the usual rule—Pseudo-Geber's books were widely read and influential among European alchemists.
The Summa Perfectionis in particular was one of the most widely read alchemy books in western Europe in the late medieval period. Its author assumed that all metals are composed of unified sulfur and mercury corpuscles and gave detailed descriptions of metallic properties in those terms. The use of an elixir for transmuting base metals into gold is explained (see philosopher's stone) and a lengthy defense is given defending alchemy against the charge that transmutation of metals was impossible.
The practical directions for laboratory procedures were so clear that it is obvious the author was familiar with many chemical operations. It contains early recipes for producing mineral acids, much like the earlier Arabic corpus. It was not equaled in chemistry until the 16th century writings of chemist Vannoccio Biringuccio, mineralogist Georgius Agricola and assayer Lazarus Ercker.
The next three books on the list above are shorter and are, to a substantial degree, condensations of the material in the Summa Perfectionis.
Two further works, Testamentum Geberi and Alchemia Geberi, are "absolutely spurious, being of a later date [than the other four]", as Marcellin Berthelot put it, and they are usually not included as part of the Pseudo-Geber corpus. Their author is not the same as the others, but it is not certain that the first four have the same author either. De Inventione Veritatis has the earliest known recipe for the preparation of nitric acid.
Arabic alchemy was held in high esteem by 13th century European alchemists, and the author adopted the name of an illustrious predecessor, as was usual practice at the time. The authorship of Geber (Jabir ibn Hayyam) was first questioned in the late 19th century by the studies of Kopp, Hoefer, Berthelot, and Lippmann. The corpus is clearly influenced by medieval Arabic writers (especially by Al-Razi, and to a lesser extent, the eponymous Jabir). The identity of the author remains uncertain. He may have lived in Italy or Spain, or both. Some books in the Geber corpus may have been written by authors that post-date the author of the Summa Perfectionis, as most of the other books in the corpus are largely recapitulations of the Summa Perfectionis. William R. Newman in his 1991 crtitical edition of the Summa perfectionis argues that the author of the Summa perfectionis was Paul of Taranto. Crosland (2004) refers to Geber as "a Latin author" while still emphasizing the identity of the author being "still in dispute". 
The estimated date for the first four books is 1310, and they could not date from much before that because no reference to the Summa Perfectionis is found anywhere in the world before or during the 13th century. For example, there is no mention in the 13th century writings of Albertus Magnus and Roger Bacon.
The degree of dependence of the corpus from actual Arabic sources is somewhat disputed. Brown (1920) asserted that the Pseudo-Geber Corpus contained "new and original facts" not known from Arabic alchemy, specifically mention of nitric acid, aqua regia, oil of vitriol and silver nitrate. Already in the 1920s, Eric John Holmyard criticized the claim of Pseudo-Geber being "new and original" compared to medieval Arabic alchemy, arguing for direct derivation from Arabic authors. Holmyard later argued that the then-recent discovery of Jabir's The Book of Seventy diminished the weight of the argument of there being "no Arabic originals" corresponding to Pseudo-Geber, By 1957, Holmyard was willing to admit that "the general style of the works is too clear and systematic to find a close parallel in any of the known writings of the Jabirian corpus" and that they seemed to be "the product of an occidental rather than an oriental mind" while still asserting that the author must have been able to read Arabic and most likely worked in Moorish Spain.
With Brown (1920), Karpenko and Norris (2001) still assert that the first documented occurrence of aqua regia is in Pseudo-Geber. By contrast, Ahmad Y. Al-Hassan (2005) claimed that Arabic texts dated to before the 13th century, including the works of Jabir and Al-Razi, did in fact contain detailed descriptions of substances such as nitric acid, aqua regia, vitriol, and various nitrates, and Al-Hassan in 2009 argued that the Pseudo-Gerber Corpus was a direct translation of a work originally written in Arabic, pointing to a number of Arabic Jabirian manuscripts which already contain much of the theories and practices that Berthelot previously attributed to the Latin corpus.