Bluestone is a generic, commercial, or cultural name for several types of blue-grayish colored rock. It may mean sandstones, slate, limestones, basalt, or diabase/dolerite rocks in various places.
Kindly note that bluestone isn’t a geological term. It may refer to any of these igneous, sedimentary, or metamorphic rock types:
1. Sandstones
Bluestones in Pennsylvania, Western New Jersey, Eastern New York, and the Canadian Appalachians refer to sandstones with a bluish hue. In New York and Pennsylvania, these rocks mean explicitly feldspathic greywacke.
These sandstones were named bluestones since those first mined in New York’s Ulster County had a deep-blue color. However, they do come in other colors, especially grays and reds. Also, weathering may further alter their colorations.
What are sandstones? Sandstones are clastic sedimentary rocks with sand-sized (0.0625 to 2 mm) silicate grains. These rocks have a rough, granular texture and account for 20-25% of sedimentary rock.
2. Limestone
In Shenandoah Valley, Virginia, bluestones refer to blue-colored limestone rocks. However, sun, rain, and other weathering agents may make the fade to light gray or have other colors.
Also, the Midnight or Belgian bluestones from Belgium’s Hainaut quarries and those from Country Carlow, Kilkenny, and Galway in New Zealand are limestones. Even the Chinese bluestones are gray limestones.
Limestone is a versatile calcium carbonate chemical sedimentary rock with mainly calcite and aragonite. It is the most common non-detrital sedimentary rock and the primary lime source.
Also, limestone is the most crushed stone in the US and has many uses in construction, paving, landscaping, concrete, the railway and roading industry, etc. More uses include making roofing granules, cement, and dimensional stone. These rocks are also used in pathways, patios, curbing, etc.
2. Basalt
Bluestones refer to basalt in Victoria, Australia, and some parts of New Zealand. Basalt is hard, fine-grained, dark-colored, extrusive igneous rocks. It is the most abundant rock on Earth, covering the ocean floor as mostly pillow lava.
Basalt is a mafic rock with mainly calcium-rich plagioclase and augite. However, it has other minerals in minor quantities and is the extrusive equivalent of gabbro. Its medium-grained version is diabase, which we will see next.
Some of the uses of the Victorian basaltic bluestones include making buildings (warehouses, building foundations, houses, etc.), walls, bridges, and railway ballast (crushed blue metal). Also, people have used them as cobblestones on gutters, curbs, etc.
On the other hand, the dark-colored Port Chalmers bluestones in New Zealand are basalts. These grey basal basalt rocks have been building materials in the past and presently in this area.
4. Dolerite or diabase
In Tasmania, Australia, and at Stonehenge, the UK, bluestones refer to the predominantly bluish to black-colored dolerite or diabase rocks.
Dolerite or diabase, officially known as microgabbro, is a medium-grained, dark-colored subvolcanic or hypabyssal rock. Its composition is like gabbro, a coarse-grained intrusive equivalent of basalt.
Subvolcanic rocks are intrusive rocks formed inside the Earth’s crust but at shallower depths. Usually, they occur in sills, dikes, and plugs.
Cooling is faster at these shallow depths. Thus, they will not have a coarse-grained texture like gabbro.
Some uses of dolerites include crushed to make aggregate and in the dimension stone industry. Additional uses include landscaping, paving, sauna stones, lining furnaces or kilns, etc.
5. Slate
In Southern Australia, bluestones refer to the slate. These are fine-grained, foliated metamorphic rocks made from clay or volcanic ashes. They have the finest grains of all metamorphic rocks.
Slate forms from low-grade and regional metamorphism of shale, a sedimentary parent rock. Depending on their composition, they come in many colors, including grays, black, beige, etc.
Some of the uses of slate include making roofing shingles, interior or exterior tiles, and wall cladding. More applications are paving walkways, pathways, and patios, making headstones, etc.
Bluestone uses
We have already mentioned the specific use of the various rocks known as bluestones. Let us quickly give you more uses.
- Aggregate or gravel: These stones are crushed to make gravel for various uses, including roading, construction, fills, concrete, asphalt, etc. Also, you may use them on unpaved walkways, patios, and pathways with larger rocks working as ripraps.
- Dimensional stone industry: Bluestones make perfect building blocks, slabs, paving tiles, roofing shingles, and ashlar veneers. Also, they make monuments, benches, steps, and wall facades, among other uses, including decorative.
- Sculptures or statues: You can use bluestones to make sculptures or statues in gardens, squares, parks, or indoors. A good example is the giant Opus 40, created in 1938 by Harvey Fite. He used bluestones from an abandoned quarry in Saugerties, NY, to make this monumental sculpture for a record 37 years. Today, this spot is an attraction to tourists and locals.
How are bluestones named?
Most bluestones get their names from points of origin. So, European or Pennsylvania bluestones mean they are from Europe and Pennsylvania, respectively. Rarely will vendors talk about the specific rock.
Therefore, I encourage you to inquire which rock it is. Otherwise, it may not suit your intended use. For instance, limestones are not suitable in areas exposed to acids.