Alabaster is a soft, fine-grained mineral prized since antiquity for its translucent quality and ease of carving. Many ancient civilizations, including the Egyptians and Mesopotamians, relied on this material to create a wide variety of objects, from small perfume vessels to large sculptural works. This beautiful stone is not a single mineral but a term that describes two distinct substances. This dual nature explains its varied appearance and geographical distribution across the globe.
The Dual Nature of Alabaster
The term “alabaster” is used commercially and historically to describe two chemically different mineral compounds. The material most commonly identified as alabaster today is a dense, fine-grained variety of gypsum, which is hydrated calcium sulfate. This type has a very low hardness, registering only 1.5 to 2 on the Mohs scale, meaning it can be easily scratched, even with a fingernail.
The second type, often called Oriental alabaster or onyx marble, is composed of calcite, which is calcium carbonate. Calcite alabaster is slightly harder, registering 3 on the Mohs scale, and cannot be scratched by a fingernail. The two types can be readily distinguished by a simple chemical test. Calcite alabaster will visibly effervesce when a drop of hydrochloric acid is applied, while gypsum alabaster remains unaffected.
Geological Processes of Formation
The two different alabaster types form through distinct geological processes. Gypsum alabaster, the sulfate variety, is an evaporite mineral, meaning it forms in arid environments where saline water bodies dry up. This process occurs in shallow basins, like lagoons or salt lakes, where the evaporation of water concentrates dissolved minerals to the point of precipitation. The calcium sulfate crystallizes out of the brine, creating thick, layered sedimentary deposits.
In some cases, the initial deposit is anhydrite, which lacks water in its structure. Over millions of years, as tectonic forces lift the material and surface waters infiltrate the rock, the anhydrite slowly rehydrates. This rehydration process transforms the anhydrite back into gypsum, forming the dense, fine-grained nodules characteristic of gypsum alabaster.
Calcite alabaster, the carbonate variety, forms through precipitation from water containing dissolved calcium carbonate. This process is most often seen in limestone caves, where the material deposits as flowstone, stalagmites, and stalactites. The slow, steady drip of water saturated with calcium carbonate leaves behind mineral layers as carbon dioxide escapes, creating the banded appearance often seen in this stone.
The same precipitation can occur when calcareous spring water reaches the surface, depositing the stone in successive layers similar to travertine. This layering often results in beautiful, swirling bands of color that are responsible for its alternate name, onyx marble. This method of deposition from flowing water gives calcite alabaster its characteristic structure, contrasting with the massive sedimentary beds of the gypsum form.
Global Sources and Deposits
The sources of alabaster correspond directly to the specific mineral type found there. The most well-known modern source of high-quality gypsum alabaster is Volterra, Italy, where the material is found in nodular masses embedded within limestone. This region remains a significant center for alabaster carving and trade in Europe.
Historically, gypsum alabaster was also extensively quarried in the Midlands of England, particularly in areas like Derbyshire, where it occurs in thick nodular beds. The Ebro Valley in Spain is also a major global supplier, with deposits originating from a prehistoric inland sea.
Calcite alabaster, historically called Oriental alabaster, was primarily sourced from the Middle East and North Africa. Ancient Egyptians used massive quantities of this stone, often extracted from quarries near Alabastron and Assiut, for sarcophagi, canopic jars, and other funerary objects. Today, significant deposits of high-quality calcite alabaster, often sold under the name Mexican onyx, are quarried in Mexico and used for various ornamental purposes.
Distinctive Physical Characteristics and Uses
Alabaster’s appeal in art and design stems from its physical properties. Its relative softness makes both types desirable for sculptors. This low hardness allows the stone to be carved easily with simple tools, enabling artists to achieve fine detail and intricate forms.
A distinguishing feature is its translucency, which allows light to pass through the stone, particularly when it is cut into thin sheets. This property creates a warm, soft glow, making alabaster an excellent material for lamps, light fixtures, and architectural elements like windows in medieval churches. While the purest form is snow-white, alabaster often contains mineral impurities, such as iron oxides, which produce a range of colors from cream and yellow to brown clouding and distinctive veining.
Due to its solubility and softness, alabaster is generally confined to indoor applications, such as statuary, ornamental vases, and decorative architectural details. The material is also valued for its fine, homogeneous texture, which can be polished to a smooth, satiny finish.