The term “bluestone” is a commercial designation used in the building and landscape industries for any natural stone exhibiting a bluish-gray color. This descriptive name is not a formal geological classification, which creates significant ambiguity depending on where the stone is quarried. The underlying geological material and composition vary dramatically across continents. To understand bluestone, one must identify the specific rock type and its geographic origin.
Geographic Variations and Geological Identity
The term bluestone lacks a singular geological definition, meaning knowing the stone’s source is the only reliable way to determine its composition and physical properties. This commercial flexibility has led to three primary geological identities across major regions.
In North America (northeastern U.S. and Canada), bluestone refers almost exclusively to sedimentary sandstone. In the United Kingdom, the bluestones (like those at Stonehenge) are igneous rocks, typically dolerite or rhyolite. In Australia, particularly Victoria, the name is applied to a dense, fine-grained volcanic rock known as basalt.
North American Bluestone: Composition and Origin
The bluestone quarried primarily in Pennsylvania and New York is a fine-grained, feldspathic sandstone, geologically classified as a greywacke. This sedimentary rock originated 370 million years ago during the Devonian Period from the erosion of the ancient Acadian Mountains. Rivers carried the sediment into the sea, forming the vast Catskill Delta.
The mineral composition is a mix of quartz, feldspar, and mica, cemented by silica and iron oxides. The signature blue-gray coloration results from the stone forming in an oxygen-poor environment, which caused iron minerals to turn green or blue. Layers exposed to oxygen during formation exhibit colors like red, brown, or lilac due to oxidized iron. Its sedimentary nature means it naturally formed in distinct layers, allowing it to be easily split into flat slabs along bedding planes.
Volcanic Bluestone: Composition and Origin
The bluestones found in the United Kingdom and Australia share a common igneous origin, formed from volcanic activity. In the UK, the Preseli bluestones of Wales (used at Stonehenge) are predominantly dolerite, a metamorphosed igneous rock, along with some rhyolite. Dolerite is chemically equivalent to basalt but cooled slightly slower beneath the Earth’s surface, resulting in a medium-grained texture.
The dolerite is composed of minerals like pyroxene and altered plagioclase feldspar. In Australia, Victorian bluestone is pure basalt, an extrusive igneous rock that cooled rapidly from surface lava flows. This rapid cooling resulted in an exceptionally dense, fine-grained structure, with a mineral makeup dominated by pyroxene, olivine, and plagioclase feldspar.
How Composition Determines Properties and Uses
The drastically different geological compositions of bluestone directly dictate their physical properties and intended uses. North American sandstone bluestone, being sedimentary, is relatively softer and more porous. Its layered structure allows it to be easily processed into thin, flat slabs known as flagstone. This makes it an ideal choice for residential applications like patios, walkways, and decorative wall veneers.
In contrast, the volcanic bluestones (basalt and dolerite) are significantly harder, denser, and less porous due to rapid crystallization from molten rock. This igneous composition makes them highly resistant to wear, compression, and weathering, often surpassing the durability of granite. These robust properties lend the volcanic variants to heavy-duty applications, such as curbing, historical monuments, and commercial paving in high-traffic areas.