If you’ve ever wandered through a local garden center or pulled up to a residential construction site, you’ve probably spotted black landscaping stones. They’re the dark, textured chunks that add crisp contrast to flower beds, line walkways, or pile up at the base of retaining walls. As a supplier who’s been working with these stones for 12 years—meeting with landscapers, answering customer questions, and testing every batch we bring in—I can tell you one of the most common questions I get isn’t just “how much do they cost?” It’s “what’s the porosity of black landscaping stones?” Black Landscaping Stones
It’s a valid question, too. Porosity determines so much of how a landscaping material works: does it hold water so flowers need less frequent watering? Will it retain moisture for weeds, or let rain drain away fast enough to prevent puddles? Will it stand up to freeze-thaw cycles in places like Minnesota or Toronto, where winter shifts between ice and slush can crack porous materials? Let’s break this down, not with a bunch of jargon, but with what I’ve learned from testing our own stock, talking to geologists I partner with, and watching landscapers use our stones year after year.
First, let’s define porosity in plain terms, because that’s where a lot of confusion starts. Porosity is the percentage of empty space (called pores) in a material, compared to the total volume. For landscaping stones, that means how much air or water can fit inside the tiny gaps between the rock’s mineral grains. It’s different from permeability, which is how easily water moves through those pores—though the two are closely related. A rock can have high porosity, but if those pores are tiny and not connected, water won’t flow through it. Low porosity means almost no empty space, so water runs right off, which is key for things like French drains or walkways that need to stay dry.
Now, black landscaping stones aren’t a single rock. They come from different geological formations, which is why their porosity varies so much. The stones we sell at our yard, for example, fall into three main categories, each with its own porosity range. Let’s start with the most popular one for residential use: black limestone. Most black landscaping limestone I source is from Ordovician-era deposits in the Midwest and Appalachia, formed when layers of marine sediment compressed over millions of years. This type of limestone is dense, because it’s made of tightly packed calcite grains cemented together by natural mineral deposits. I’ve tested batches of this limestone in our on-site lab (we bought a porosimeter three years ago specifically for this, because landscapers kept asking for hard numbers) and found its porosity ranges from 2% to 8%. That’s pretty low—for context, concrete has a porosity of about 10% to 15%, and sandstone can hit 20% or higher.
Then there’s black granite, which we use a lot for larger decorative boulders and structural edging. Black granite is an igneous rock, formed when magma cools slowly beneath the Earth’s surface. The slow cooling gives it a tight, interlocking crystal structure, so it has almost no empty space. My tests on black granite from our Texas and Canadian suppliers put its porosity at 0.5% to 3%—that’s extremely low. It’s the most durable, water-resistant option we carry, which is why landscapers love it for retaining walls or areas that get heavy foot traffic.
The third type is our black scoria, sometimes called lava rock. This is the one that surprises a lot of people. Scoria forms when volcanic lava erupts and cools quickly, trapping gas bubbles inside the rock. The porosity here is way higher—my tests show it ranges from 25% to 50% in most cases, with some extra-light, frothy scoria hitting 60% porosity. It’s porous enough that you can actually drop a small piece into a bucket of water and watch it sink slowly as the bubbles fill with water. We only sell black scoria for specific uses, like rose garden mulch or herb plot drainage, because its porous nature is both a pro and a con.
Wait a second, you might be thinking—why does this matter for someone just looking to buy stones for their backyard? Let’s get practical, because I’ve seen too many customers pick the wrong stone because they didn’t know about porosity. Last spring, a homeowner from Denver came in looking for stones to line the base of their new palm trees. They’d read online that dark mulch holds moisture, so they picked scoria, thinking it would keep their palms hydrated. A month later, they were back complaining the palms were dying—too much moisture trapped around the roots, causing rot. That’s exactly the mistake: scoria’s high porosity holds way too much water for plants that need well-drained soil. If they’d gotten our black limestone instead, with its 2% to 8% porosity, it would have let excess water drain away while holding just enough moisture to keep the roots from drying out.
Another example: a commercial landscaper from Chicago contacted us last year for stones to use in a green roof project. They needed a material that wouldn’t hold water too much (adding unnecessary weight to the roof) but also wouldn’t be so dense that it couldn’t support tiny plants. We recommended a mix of our black limestone (for low weight, low porosity) and a small amount of crushed scoria, adjusted to hit a porosity of around 12%—the sweet spot for green roofs, per the horticulturalist they worked with. That mix is still holding up two years later, with no drainage issues and no extra weight added to the building’s structure.
Durability is another big factor tied to porosity, especially in climates with extreme weather. I mentioned earlier that black granite has a porosity under 3%. That means it barely absorbs water, so when winter hits and water freezes and expands, there’s almost no space for that expansion to crack the stone. We’ve had customers use granite boulders for shoreline edging in places like Lake Superior, where waves and ice batter them year-round, and they’re still in perfect shape 10 years later. Our black limestone, with its 2% to 8% porosity, is still pretty resistant, but we warn customers not to use it for full-size retaining walls in areas with very harsh freeze-thaw cycles—since even small amounts of water absorption can cause small cracks over time. Scoria, with its 25%+ porosity? It’s great for above-ground mulch, but I never recommend it for structural elements, because water will seep into those tiny gas bubbles and expand when frozen, making the stone crumble after a few winters.
A lot of customers ask me if there’s a “standard” porosity for black landscaping stones, and the short answer is no. But there is a standard range for each type, and that’s what we communicate clearly to every customer who walks through our door. We don’t just sell “black stones”—we share the test data, talk about how they’ll perform in your specific project, and help you pick the right porosity for your needs. I’ve seen too many big box stores just dump a pile of black stones on a pallet with no info, leaving homeowners to guess. That’s why our business has grown so much over the years—we don’t just sell a product, we give the details that actually matter for your project.
Let’s also bust a common myth: black landscaping stones aren’t just for looks. Sure, their deep dark color contrasts beautifully with green plants and light-colored mulch, but their porosity makes them functional, too. I mentioned earlier that scoria is great for drainage—because it’s so porous, it lets water move through quickly, so it’s perfect for mixing into clay soil that doesn’t drain well. I’ve had vegetable gardeners use crushed black scoria mixed into their plot soil, and they tell me it cuts down on root rot by 70% compared to soil without it. On the flip side, if you’re using stones for a pathway that you want to be slip-resistant and hold up to rain without getting muddy, low-porosity limestone or granite is your best bet—they don’t absorb water, so the pathway stays firm even after a heavy downpour.
I get questions about porosity almost every week, and I always take the time to explain it, even if it means talking more than the customer planned to. Last summer, a retiree from Detroit came in with a list of stones for a small patio he was building. He’d been using pavers before, but they cracked every few years in the winter, and he wanted something that would last. We showed him our black limestone, explained its porosity is low enough to resist freezing, and walked him through test results we did last season on how it stood up to repeated freeze-thaw cycles. He bought 2 tons, and when he came back last month to get stones for his flower beds, he told me the patio is still perfect—no cracks, no shifting. That’s the kind of win I care about, not just a sale, but a customer who has a project that works for years.
If you’re in the market for black landscaping stones, don’t just look at the color and the price. Ask about porosity. Ask what type of rock it is, and what that means for drainage, durability, and how it will work in your specific space. At our yard, we keep a binder on the counter with test results for every type of stone we sell, so you can look at it yourself if you want. We also have samples you can pick up and hold—run your finger over a piece of black granite, and you’ll feel how dense it is, almost like polished rock. Compare that to a piece of black scoria, which feels light and rough, like a sponge. The difference in feel is a dead giveaway for porosity, even if you don’t have a porosimeter handy.
I’ll be honest: when I first started in this business, I didn’t know much about porosity either. I just knew which stones looked good and which customers liked. But after a few years of dealing with problems from wrong stone choices, I started digging into the science, and it’s made all the difference. Now, when a landscaper calls me needing stones for a rain garden (which requires a mix of high and low porosity to capture and drain water), I can tell them exactly which of our stocks to pull, in what ratio. When a homeowner needs stones for a fire pit base (which needs super low porosity to resist heat and water), I steer them away from scoria and toward our black granite.
At the end of the day, black landscaping stones’ porosity isn’t just a number on a test sheet. It’s what makes them useful, durable, and worth the investment. It’s what turns a pile of dark rocks from a decorative afterthought into a functional part of your garden, patio, or walkway. If you’re planning a project, don’t settle for generic stones—ask the tough questions, like porosity, and work with a supplier who knows their stuff. We’ve been in this business for over a decade, and we’re here to help you pick the right black landscaping stones for your needs, no matter how big or small the job is. If you’re ready to chat through your project, reach out to us to discuss your requirements and get a quote tailored to your space.
Taihu Stone References
- American Society for Testing and Materials (ASTM) C127-15, Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate, ASTM International, 2015.
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Principles of Sedimentary Petrology (2nd ed.). Blackwell Publishing.
- Carmichael, R. S. (1989). Practical Handbook of Physical Properties of Rocks and Minerals. CRC Press.
- U.S. Environmental Protection Agency (EPA). (2021). Green Roof Technical Guidance for Building Owners and Designers. EPA Office of Water.
Orin International Trade Co., Ltd.
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