Although marketed as “angel skin opal,” “rock wood,” and “mountain leather,” palygorskite is neither opal, wood, nor leather. This unusual, parchment-like mineral can be cut into cabochons or carved.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Monoclinic and orthorhombic, with many polytypes. Crystals very elongated, in bundles; usually thin flexible sheets made up of complexly intergrown fibers, resembling parchment or leather.|
|Colors||White, gray, rose pink, pale pink, yellowish.|
|Hardness||Pure material very soft (around 2): angel stone is impregnated with silica and hardness = 4.5.|
|Specific Gravity||2.21 (pure); angel stone is 2.1-2.2.|
|Cleavage||Easy; not observed on impregnated material.|
|Optics||Around 1.55 (probably reading values for quartz impregnation) for angel stone.|
|Etymology||Palygorskite is named after the type locality, Palygorskaya, in the Urals, Russia. Attapulgite is named after Attapulgus, Georgia, USA.|
|Occurrence||Palygorskite is a product of alteration and occurs in hydrothermal veins, serpentines, and granitic rocks.|
Palygorskite is closely related to sepiolite, also known as meerschaum. Structurally, it contains amphibole-like silicate chains. According to a 2010 study, intermediate compositions exist between aluminum/magnesium-rich palygorskites and magnesium-rich sepiolites.
Palygorskites typically contain parchment-like, matted aggregates of fibrous crystals. This material can look and feel like flexible leather. Due to this quality, it’s received fanciful designations such as “rock wood,” “mountain wood,” and “mountain leather.” (Some compare it to wet newspaper).
The “angel skin” of this odd gem material can show very attractive pale gray, pink, and yellow colors. However, the appellation “angel skin opal” is a misnomer. Palygorskites and opals are altogether distinct gem species. Nevertheless, crystal inclusions of this mineral have been found in pink opals.
“Angel stone” is a microcrystalline material made principally of pale, pink palygorskite impregnated with silica. This makes the material harder (4.5) and more durable than pure palygorskite for jewelry use.
Attapulgite refers to palygorskite sourced from Attapulgus, Georgia. This synonym appears more frequently in medical literature.
In addition to its appeal as an unsual gemstone, this mineral has many commercial uses due to its liquid absorbency. Its practical applications range from pet litter and drilling mud to anti-diarrheal medications. Historically in the US, ranchers used this mineral for fulling, absorbing fat from wool. Hence, it received the name, “fuller’s earth.” The ancient Mayans used this mineral to create the color known as Maya Blue.
- United States: Attapulgas, Georgia; Virginia; Metalline Falls, Washington.
- England; France: Morocco; Russia; Scotland.
- Mexico; Peru (angel stone).
Even after its silica impregnation, angel stone remains relatively soft compared to more common jewelry stones. Store any palygorskites separately from other gems to avoid contact scratches and keep away from moisture.
See our Gemstone Jewelry Cleaning Guide for care recommendations.