Granite, marble, and slate: what your stone is made of

Most people don't know what their family headstone is made of, and there's no reason they should — until it's time to clean it. South Shore cemeteries hold a mix spanning nearly four centuries, and the material matters more than almost anything else in deciding how a stone should be treated.

Granite: the modern standard

Most markers set in the last 60 to 80 years are granite — dense, hard, and about as durable as monument stone gets. Polished granite has a mirror-like sheen on the face; natural or "rock-pitched" granite has a rougher, unpolished texture. Both hold up to weather extremely well, but both are porous at a microscopic level, which is how lichen and algae get a foothold and how salt and moisture work their way in near the coast.

Granite tolerates a stone-safe biological cleaner well, but it does not tolerate abrasion. A wire brush or an aggressive scrub pad will leave fine scratches that dull the polish permanently — the kind of damage that shows up as a hazy patch you can't buff back out.

Marble: softer, and more common on older stones

Marble was the material of choice through much of the 1800s and into the early 1900s, before granite cutting became widely affordable. It's noticeably softer than granite and made of calcium carbonate, which reacts with acid — including the mild acidity in rain. Over a century or two, that reaction is what produces the smooth, "melted" look on old marble markers, where carved lettering has gone shallow or vanished entirely.

Because marble is acid-sensitive, this is where household cleaners do the most damage. Vinegar, most all-purpose sprays, and anything acidic will etch the surface on contact. Marble needs a pH-neutral approach, gentler pressure, and more patience — the same biological solution used on granite works, but the timeline to see a difference is usually longer.

Slate: the oldest South Shore stones

Slate shows up on the oldest markers in South Shore burying grounds — thin, dark, often hand-carved stones from the 1600s and 1700s, sometimes with the winged skulls or willow-and-urn motifs typical of that period. Slate is fine-grained and holds carving detail remarkably well for its age, but individual stones vary a lot in stability. Some are structurally sound; others have hairline delamination — the layers of the stone beginning to separate — that isn't obvious until pressure is applied to the surface.

On slate, the first step is never cleaning — it's checking whether the stone can safely take any treatment at all. We test stability before anything touches the surface, and on anything that shows signs of delamination or looseness, we work far more conservatively than we would on a modern granite marker.

Why we use the same product across all three

D/2 Biological Solution — the cleaner used on national monuments and military cemeteries — is biodegradable and pH-neutral, which is why it's safe across granite, marble, and slate alike, unlike a pressure washer or a household cleaner that might be fine on one material and destructive on another. What changes between materials isn't the product; it's the pressure, the number of visits, and how conservatively we treat a fragile antique stone versus a modern polished one.

If you don't know what your family stone is made of

That's normal, and it's not something you need to figure out yourself. Tell us the cemetery and roughly where the plot is, and we'll identify the material, check its condition, and tell you honestly what it needs — whether that's a standard three-visit cleaning or a slower, more careful approach for an older or more fragile stone.

All three materials sometimes sit within a few hundred feet of each other. Cohasset Central Cemetery is the clearest example we work in — documented eighteenth-century slate by the North River carvers, Victorian marble, and modern granite in one 4.5-acre burying ground.