Bluestone is most commonly used as a natural stone for patios, walkways, and outdoor living spaces, prized for its durability and blue-gray color. But the word “bluestone” has more than one meaning. It also refers to copper sulfate (a blue crystal used in agriculture and water treatment) and to the famous prehistoric stones of Stonehenge. Here’s a closer look at each.
Patios, Walkways, and Outdoor Surfaces
The most popular use of bluestone today is in residential and commercial landscaping. It’s a dense sandstone or flagstone, typically quarried in the northeastern United States, that works well for patios, walkways, pool decks, steps, driveways, and retaining walls. Many bluestone sidewalks installed over a century ago in cities like Philadelphia and Boston are still in service, which speaks to how well the material holds up over decades of foot traffic and weather.
Bluestone comes in two main finishes, and the one you choose affects both the look and the installation process. Natural cleft bluestone is split along the stone’s natural internal layers, producing a textured surface with slight high and low points. That uneven texture gives it excellent slip resistance, making it a strong choice for pool surrounds or areas that get wet. Because it breaks naturally, thickness varies: a piece labeled 1 inch typically ranges from 3/4 inch to 1 1/4 inches, which makes installation a bit more involved.
Thermal bluestone, on the other hand, is saw-cut to an exact thickness and then flamed with a blowtorch to create a light surface texture. The result is a smoother, more uniform look that suits modern or minimalist designs. Because the pieces are consistent in size and squareness, thermal bluestone is generally easier to lay than natural cleft. Despite its smoother appearance, the flaming process still leaves enough texture for reliable traction underfoot.
Caring for Bluestone Surfaces
Bluestone is low-maintenance compared to many other natural stones, but sealing is recommended to protect against stains. For the best protection, seal the stone on all six sides at least 24 hours before installation. After that, routine cleaning with a pH-neutral stone cleaner is usually all you need. Avoid acidic cleaners, which can react with natural stone and cause discoloration or surface damage. Periodic deep cleaning with a non-abrasive stone cleaner handles stubborn dirt, and resealing every few years keeps stain resistance at its peak.
Copper Sulfate in Agriculture and Water Treatment
“Bluestone” is also an old name for copper sulfate, a bright blue crystalline compound with a wide range of agricultural and industrial uses. Farmers and groundskeepers use it as a fungicide, algaecide, herbicide, and root killer. Mixed with calcium hydroxide, it becomes Bordeaux mixture, a classic treatment for fungal diseases on grapevines and fruit trees that’s been in use since the 1800s.
Beyond the farm, copper sulfate shows up in water treatment (killing algae in reservoirs and pools), in fertilizers, and across industries including textiles, leather tanning, wood preservation, battery manufacturing, and paint production. Its anhydrous form also works as a drying agent in chemical labs.
Safety Concerns With Copper Sulfate
Copper is an essential nutrient in small amounts, but copper sulfate is toxic if swallowed, inhaled, or handled carelessly. Ingesting large amounts can cause severe abdominal pain, vomiting, diarrhea, and jaundice. In serious cases, it leads to kidney failure, liver failure, and death. Breathing in copper dust or fumes can trigger metal fume fever, with symptoms including chest pain, chills, cough, and a metallic taste in the mouth. Long-term inhalation exposure can cause permanent lung scarring and reduced lung function.
In parts of southern Africa, copper sulfate has a long history in traditional medicine. A study of 201 traditional healers found that 72% used it for healing purposes, administering it for skin rashes, pain and swelling, and sexually transmitted infections. It was given to patients of all ages, including infants. Researchers flagged this as a significant safety concern given the compound’s toxicity, and called for tighter regulation of its use in traditional practice.
The Bluestones of Stonehenge
In archaeology, “bluestones” refers to the 42 smaller megaliths at Stonehenge, each a type of igneous rock that appears bluish when wet or freshly broken. Geologists traced these stones to the Preseli hills in Pembrokeshire, west Wales, roughly 180 miles from Stonehenge. The largest source is an outcrop called Carn Goedog on the north slope of the hills, which supplied the monument’s spotted dolerite (an igneous rock flecked with white spots). A second outcrop, Craig Rhos-y-felin, provided at least one type of rhyolite found at the site.
Charcoal recovered from quarry platforms at both outcrops dates to around 3000 BC. Archaeologists from the University of Southampton found that the rock faces naturally form vertical pillars, and Neolithic workers could pry individual pillars loose by driving wedges into the joints between them. They then lowered each pillar onto a stone-and-earth loading platform and dragged it away on wooden sledges. The current theory is that Stonehenge originally consisted of a circle of rough, unworked bluestone pillars, and the larger sandstone blocks were added about 500 years later.
What makes the bluestones remarkable is the distance they traveled. Every other known Neolithic monument in Europe was built with megaliths sourced from within about 10 miles. The 180-mile journey from Wales to Wiltshire suggests the stones carried deep symbolic meaning, possibly connecting early farming communities in Wiltshire to ancestral lands in Wales. The quarries’ location on the north side of the Preseli hills also casts doubt on the long-held theory that the stones were floated by sea along the south Welsh coast, pointing instead to an overland route across what is now southern England.