The term “blue stone” is a broad descriptive label applied to various materials, leading to confusion across fields like geology, commerce, and chemistry. It refers to materials ranging from construction aggregates and archaeological artifacts to prized gemstones and specific chemical compounds. Clarifying the most common interpretations used in modern and historical contexts is necessary to understand what people mean when they use the term.
Bluestone as a Geological Trade Material
In North America, the term “Bluestone” most commonly refers to a durable type of sandstone, specifically a feldspathic greywacke, quarried primarily in Pennsylvania and New York. This sedimentary rock originated from the Catskill Delta formation, where sediments eroded from the ancient Acadian Mountains were deposited about 370 to 345 million years ago. The layered deposition of these sands and silts allows the rock to be naturally split into flat, usable slabs, making it highly valued in the construction industry.
The characteristic blue-gray color is a result of the rock’s mineral composition and the oxygen-poor conditions during its formation. However, despite the name, the commercial material appears in a wide spectrum of colors, including shades of green, lilac, brown, and gray. It is widely used as dimensional stone for patios, walkways, curbing, and architectural cladding due to its exceptional durability and resistance to weathering. Its ability to naturally cleave into flat pieces makes it a popular choice for outdoor paving applications.
Across the Atlantic, particularly in Europe, the name “Bluestone” is applied to different geological materials used for similar commercial and architectural purposes. In Belgium and Ireland, the designation often refers to a dense, dark limestone. In the United Kingdom, it may refer to various volcanic or igneous rocks like dolerite. This difference highlights the term’s commercial nature, where “bluestone” describes a functional, hard, often dark-colored building material rather than a specific mineral composition.
Identification of Major Blue Gemstones and Minerals
The most prominent example is the Sapphire, a gem-quality variety of the mineral corundum. Sapphire is renowned for its exceptional hardness, scoring a 9 on the Mohs scale, second only to diamond. Its intense blue color is caused by trace amounts of iron and titanium impurities within the corundum crystal lattice. The finest specimens are often faceted to maximize their brilliance and clarity, yielding a high-value gemstone.
In contrast, Lapis Lazuli is not a single mineral but a metamorphic rock composed mainly of the mineral lazurite, along with calcite and pyrite. Lazurite provides the deep, intense ultramarine blue for which the stone is known. The small, scattered inclusions of golden pyrite and white calcite veins are characteristic features that distinguish Lapis Lazuli. Because it is a rock aggregate, Lapis Lazuli is softer, registering between 5 and 6 on the Mohs scale, and is typically cut into cabochons, beads, or used for carvings.
Another widely recognized blue stone is Turquoise, a hydrous phosphate of copper and aluminum. The copper content is responsible for its distinctive sky-blue to greenish-blue color, and it often exhibits a dark web-like pattern, known as the matrix, which is the remnant of the host rock. Turquoise is opaque and relatively soft, with a Mohs hardness ranging from 5 to 6, and has been valued as an ornamental stone for thousands of years.
Less common but notable blue minerals include Azurite and Sodalite. Azurite is a basic copper carbonate known for its deep, azure-blue color, which is often mistaken for Sodalite. However, Azurite is quite soft, with a Mohs hardness of only 3.5 to 4, making it unsuitable for most jewelry. Sodalite, a sodium chloro-aluminosilicate, is a mineral that forms with a royal blue color, often displaying contrasting white streaks from calcite impurities. It is slightly harder than Lapis Lazuli, with a Mohs score of 5.5 to 6, and is frequently used for ornamental objects and cabochons.
Historical Significance and Chemical Designations
The name “bluestone” carries significant archaeological weight in the United Kingdom, specifically relating to the construction of Stonehenge. The smaller stones within the famous megalithic monument are known as the Preseli Bluestones to distinguish them from the larger Sarsen stones. These were igneous rocks, primarily spotted dolerite and rhyolite, which were transported from the Preseli Hills in Wales, a distance of about 140 miles. The effort required to move these stones suggests they held profound cultural significance for the Neolithic builders.
A completely separate and historical interpretation of the name exists within chemistry and agriculture, where “Blue Stone” or “Blue Vitriol” refers to the synthetic compound Copper (II) Sulfate pentahydrate. This inorganic salt forms vivid blue, triclinic crystals. Historically, this chemical was used as a fungicide, an algicide, and a mordant in dyeing textiles. The name relates purely to its appearance as a blue crystalline solid, unconnected to any natural rock or mineral aggregate.