(Note: this short post was originally made on the /r/Hermeticism subreddit, which I’ve decided to copy here for posterity.)
So, there’s the “Emerald Tablet”, and then there are the “Emerald Tablets of Thoth the Atlantean”. The two are emphatically not the same thing.
The “Emerald Tablet” is a short, cryptic, almost poetic summary of early alchemical belief, and comes from the Book of the Secrets of Creation (Kitāb sirr al-ḫalīqa) attributed to Apollonius of Tyana (aka Balīnūs), which was written no later than the 11th century CE, and while there are theories that it is an Arabic translation of an earlier (no longer extant) Greek work, we don’t yet know for certain whether it was a translation of an earlier work or whether it was an original composition in Arabic. The book as a whole is an encyclopedic treatment of many things, not least of which were alchemical concoctions and magical talismans, and in this book is a vignette that shows how a narrator entered into a tomb of Hermēs Trismegistos and encountered the Emerald Tablet. This little cryptic text was then translated repeatedly into Latin and other languages, though the whole of the Book of the Secrets of Creation has rarely received any such treatment on a wide scale besides one translation into French in 1798 by Antoine Isaac Silvestre de Sacy. Some scholars put the writing of the “Emerald Tablet” (along with some of the content of the Book of the Secrets of Creation) as having been written no earlier than 600 CE and generally no later than 750 CE, with evidence suggesting that it itself (regardless of the rest of the Book) was written originally in Arabic. Due to its brevity and cryptic nature, it’s long captured the attention and imagination of many generations of alchemists and magicians. You can find a variety of translations into English on this Sacred Texts page, some of which are more fantastical than others, and M. David Litwa also includes two translations of it in his Hermetica II (containing plenty of Hermetic texts, excerpts of texts, and testimonia from sources other than the more famous Corpus Hermeticum and Asclepius texts).
The “Emerald Tablets of Thoth the Atlantean” are a channeled text, supposedly a “translation” of a 36,000 year old text put out by one M. Doreal (founder of the “Brotherhood of the White Temple”, who claimed to receive the channeled text directly from “Thoth the Atlantean”) in 1939. Unlike the “Emerald Tablet” itself or the Book of the Secrets of Creation more generally, it bears little to nothing in common with any classical or even post-classical (medieval or Renaissance) Hermetic text, whether philosophical or alchemical or astrological or magical, and instead bears many hallmarks of it being heavily influenced (if not a product of) Theosophy and New Age beliefs generally, especially those of the “Brotherhood of the White Temple” (which were mythologically based on the “Emerald Tablets of Thoth the Atlantean”, according to the organization itself, but were more likely written to codify them and give them a claim to spiritual legitimacy). In many ways, the “Emerald Tablets of Thoth the Atlantean” are a New Age parallel to the Book of Mormon, supposedly a translation from a now-lost language from a now-lost original text.
Suffice it to say that the “Emerald Tablet” (singular) is something Hermetic, in one sense or another, but the “Emerald Tablets” (plural) are not. As a result of a lot of New Age ideas being developed in the wake of 19th and early 20th century Egyptomania combined with a lot of pseudoscience and fringe (to the point of extremist) beliefs, Doreal naming his work “Emerald Tablets of Thoth the Atlantean” was certainly inspired by or derived from the much older (and much more extant) “Emerald Tablet”, although they are not related at all to each other. In many ways, it is much the same sort of deal with the “Emerald Tablets” as it is with the Kybalion.
To be fair, Thōth is important for us, as the Egyptian god who was syncretized with the Greek Hermēs as Hermēs Trismegistos, and many important works that we here also devote time and energy to studying are attributed to Hermēs Trismegistos (hence why this is “Hermeticism”). That said, there is easily a lot of stuff that isn’t Hermetic that makes use of the name or form of Hermēs or Thōth, especially the more freewheeling new age stuff that draws a lot of attraction, so there is unfortunately a good amount of confusion out there, too.