I know that this is really lignite, or black amber, but it seems like the etymology of this word must have been something like:

  1. Some mineralogist uses the term "jet" to refer to lignite.
  2. A second mineralogist, from The South, and having a very thick accent (those of you who have lived below the Mason-Dixon Line know what I am talking about) picked up the word and began to use it.
  3. Now, a third mineralogist comes along (who knew there were even this many mineralogists in the world?), this one being a Yankee. He hears the Southerner saying what he hears as "jay-et", which, if he were not so ignorant of his southerly brother's ways, he would know was equivalent to "jet".
  4. This third mineralogist then writes this word in some publication, yadda yadda yadda, the word "Jayet" is added to the dictionary

Let that be a lesson to you: Do not communicate vocally!