Goethite amethyst, also wrongly called cacoxenite amethyst
Someone once labelled these inclusions as cacoxenite because of a vague resemblance to that mineral, but amethyst with cacoxenitemay not exist at all.
It has been historically called cacoxenite in Brazil. It was not till much later that an empirical study was done and by that time it was too late to change the marketing machine and people’s habit of calling it cacoxenite.
Goethite inclusions form very typical bundles of yellow to brown needles that look a bit like a broom.
Goethite amethyst origin
This material is typical of the amethyst that comes from around Ametista do Sul, formerly called Sao Gabriel, Rio Grande do Sul, Brazil.
Goethite is an iron oxyhydroxide containing ferric iron. It is the main component of rust and bog iron ore. Goethite’s hardness ranges from 5.0 to 5.5 on the Mohs Scale, and its specific gravity varies from 3.3 to 4.3. The mineral forms prismatic needle-like crystals, needle iron ore, but is more typically massive.
Feroxyhyte and lepidocrocite are both polymorphs of the iron oxyhydroxide FeO(OH) which are stable at the pressure and temperature conditions of the Earth’s surface. Although they have the same chemical formula as goethite, their different crystalline structures make them distinct minerals.
Additionally, goethite has several high-pressure and high-temperature polymorphs, which may be relevant to the conditions of the Earth’s interior.
Goethite amethyst quartz
Amethyst is a purple variety of quartz and owes its violet color to irradiation, impurities of iron and in some cases other transition metals, and the presence of other trace elements, which result in complex crystal lattice substitutions. The hardness of the mineral is the same as quartz, thus making it suitable for use in jewelry.
Amethyst occurs in primary hues from a light pinkish violet to a deep purple. Amethyst may exhibit one or both secondary hues, red and blue.
Of very variable intensity, the color of amethyst is often laid out in stripes parallel to the final faces of the crystal. One aspect in the art of lapidary involves correctly cutting the stone to place the color in a way that makes the tone of the finished gem homogeneous. Often, the fact that sometimes only a thin surface layer of violet color is present in the stone or that the color is not homogeneous makes for a difficult cutting.
Goethite amethyst from Brazil