Jacinto de Compostela

In Spanish mineralogical literature, the name is traditionally used exclusively for the red "floater" variety of authigenic quartzes from continental gypsum-bearing marls of the Triassic Keuper formation. (They may also be found occasionally in younger Tertiary or Quaternary sediments, but in these cases the crystals were eroded out of the Keuper F.) Two types are found: 1. Isolated biterminated crystals, 2. hedgehog-like spheroidal aggregates. Both characteristically occur as "floaters" embedded in marl, or as inclusions in gypsum masses or, less commonly, aragonite crystals. It used to be thought that "Jacinto"-type quartzes were colored by hematite, but it has been determined that the color is dominantly due to clay mineral inclusions. (Gil Marco, J., 2013) (Alleged brown "jacintos" are colored by organic matter and should not be classified here. Some of the brown ones have also been mis-classified as "smoky quartz".)