Many collectors value the rosette formations of gypsum.
Gypsum is a widespread mineral, but when it occurs in a specific environment it can form into "rosettes." These spectacular formations occur in many different regions, from the Mammoth Caves of Kentucky to the briny sabkhas of Saudi Arabia. Rosettes take three basic forms: those that actually look like small carved roses, others that look like bladed spheres, and clumps of gypsum rosettes that are reminiscent of underwater coral formations.
What Gypsum Is
Gypsum is a very common mineral made of hydrated (or hydrous) calcium sulfate, which is calcium sulfate that contains water. It comes in a variety of colors and formations, and some variations fluoresce yellow under UV light. Some may even glow for a few moments after the UV light is shut off. There are many commercial uses for gypsum, including plasters, cements, ceramics and as an additive to neutralize alkaline soils.
Environment
Gypsum rosettes require a specific environment in order to form. They need an arid climate with a source of hydrated calcium sulfate, and a water table that rises and falls seasonally. They take thousands of years to form, but this is nothing on a geological timetable. Formation can take place in myriad areas, such as in sand fairly close to the surface (around two to four feet), along the banks of floodways that carry water rich in calcium sulfate or in caves that are now dry but have flooded seasonally in the past.
Formation
The rosettes of Saudi Arabia form when water rich in calcium sulfate rise and fall against a sandy surface. As the water table fluctuates, the water deposits layer upon layer of calcium sulfate into cracks and fissures in the sand. It is the cracks and fissures of the sand that , over time, produce the distinctive pattern of rosette growth as hydrated calcium sulfate is deposited. Those formations found in the Americas tend to be more crystalline, having developed in clay, so they lack the sand inclusions of the rosettes found in Saudi Arabia and the Sahara Desert.
Size
Typically, individual rosettes are anywhere from under an inch wide to 8 to 10 inches wide, with the larger individual rosettes being rarer. The largest known examples are less than 2 feet wide and are found in coarser sands and often below the water table. Clusters of individual rosettes can be huge--large enough to fill a room in some cases, although these are more often found in caves as vast formations that have been left alone for a long time.
Color
Gypsum rosettes form in a wide range of colors depending on the other geological properties of the area. They surround whatever loose sediment is near them, and this greatly influences their color and transparency. Some are nontransparent and look like bits of carved stone because of the large number of sand inclusions. Others contain yellow sulfur inclusions, and many are white from clay or other minerals. Some are entirely transparent and very crystalline in structure.
Tags: calcium sulfate, individual rosettes, look like, Saudi Arabia, water table
