Amethyst Stalactites: A Rare and Mysterious Formation

What is Amethyst Stalactite, and How Did It Form?

Amethyst “stalactite” is a tube agate that forms in the center of an agate/amethyst vug within a hydrothermal igneous formation.  It is not a limestone nor a cave formation. The agate center is formed when the chemicals and minerals in the water change as it runs through the cavity. The crystals of Amethyst then grow around the Agate.

Amethyst is a purple form of crystallized quartz. The colour comes from the combination of trace amounts of iron and aluminium, plus irradiation. Amethyst has a trigonal crystal structure, a hardness of 7 and its chemical formula is Silicon Dioxide - SiO2.

How does Amethyst stalactite form?

Amethyst "stalactites" are not actually stalactites in the traditional sense. Stalactites are formed when water containing dissolved minerals drips from the ceiling of a cave, leaving behind a deposit of mineral matter. Amethyst "stalactites" are formed differently. They are actually tube agates that form in the center of an agate/amethyst vug within a hydrothermal igneous formation.

A hydrothermal igneous formation is a type of rock that is formed when hot, mineral-rich water cools and solidifies. The water in these formations often contains dissolved silica, which can form agate. If the water also contains iron, the agate can be colored purple, which is how amethyst "stalactites" are formed.

Amethyst Stalactites: A Rare and Mysterious Formation
Amethyst Stalactites From Rio De La Plata, Artigas, Uruguay.
Photos: StalactitefromUruguay/IG

The process of forming an amethyst "stalactite" begins when a small crack or void forms in the rock. Water containing dissolved silica and iron seeps into the void and begins to cool and solidify. As the water cools, the silica molecules form agate crystals. The iron molecules also form crystals, but they are much smaller than the agate crystals. This gives the agate a distinctive purple color.

Over time, the agate crystals continue to grow, forming a tube-like structure. The tube can grow quite long, sometimes reaching several feet in length. When the tube is cut in cross-section, it reveals a beautiful radial pattern of agate crystals.


Here are some additional details about how amethyst stalactites form:

  • The water that seeps into the cracks in the ground is often rainwater that has been filtered through soil and rocks. This filtering process removes impurities from the water, leaving behind the silica.
  • The silica is deposited on the walls of the crack in a process called "supersaturation." This means that the water is holding more silica than it can normally dissolve. As the water evaporates, the silica is forced out of solution and deposited on the walls of the crack.
  • The rate at which amethyst stalactites form depends on a number of factors, including the temperature of the water, the amount of silica in the water, and the size of the crack. Stalactites can form over a period of years or even decades.


Amethyst Stalactites: A Rare and Mysterious Formation
Amethyst Stalactites from New Mine In Artigas Uruguay.
Credit: Nowar Minerals Inc.

Amethyst "stalactites" are rare geological formations, and they are only found in a few places around the world. The best known locations for amethyst "stalactites" are Uruguay, Brazil, and Morocco.

Amethyst stalactites are a beautiful and rare natural formation. They are often cut and polished into jewelry or other decorative objects.

Next Post Previous Post