Phyllite – 560 million years in the making


Slate is the result of hundreds of millions of years of geology.

Its looks, and its ability to retain strength and durability even when split very thin, have made it a popular roofing material for centuries – in every region of the world, you’ll find roofs made of slate.

But what happens if slate is left underground for even longer? If it isn’t extracted, and subjected to the incredible heat and pressure of the earth’s core for millions of years more?

The answer is phyllite.

A step on from slate

Riverstone Phyllite stone

Phyllite is the next rung along on the geological ladder. Originally, hundreds of millions of years ago, it was a soft rock like shale or mudstone.

Then it was buried, and over almost unimaginable lengths of time, it became slate. But then those same geological processes continued, and the slate became something else.

Heat, pressure and chemical activity turned the clay minerals inside it into flake-shaped mica minerals. Those mica minerals gradually enlarged, and were forced into parallel alignment.

The geological sweet spot

We call the result ‘phyllite’. The word stems from the Greek word ‘phyllon’, or leaf, which seems a strange name until you look at it up close – it’s got a greenish tinge, and can be split extremely thin.

Riverstone Quarry is situated in the San Luis region of central Argentina

But the bottom line from a construction perspective is that phyllite is a fantastic roofing material.

Really, it exists in the perfect geological sweet spot. The extra millennia have made it harder and stronger than slate. But it’s not yet become so hard that it’s impractical for roofing purposes.

Its mica flakes, and the tiny crystals on its surface, give it an unmistakable silky appearance – known in the industry as a phyllitic sheen – and its attractive grey-green colour instantly distinguish it from other products on the market.

These are just some of the reasons that make phyllite one of the world’s most sought-after architectural stone products. But the next question is – how do you get hold of it?

Click here to request a free sample of Riverstone Phyllite.

Guaranteed for a hundred years

Phyllite is quarried in only a few locations around the world. That means getting hold of it can be difficult.

But at SSQ, we’re fortunate enough to own one of those locations – the San Luis quarry in La Repressa.

It’s how we’re able to guarantee the quality of our own Phyllite, which we call Riverstone. We’ve had it extensively tested, with impressive results. Assessed to ASTM C406 standard, Riverstone was awarded an S1 rating, indicating it will last a minimum of 75 years.

It’s also been tested to see how reactive the iron oxides on its surface are. Highly reactive iron oxides lead to discoloration and leaching over time. Riverstone, on the other hand, received T1 classification – meaning it’s highly unlikely that any leaching will occur.

It’s for that reason that Riverstone comes backed with some of the most extensive guarantees in the industry. Choose Riverstone, and we’ve got you covered for a hundred years.

Click here to download the complete Riverstone brochure

Versatile, and used all over the world

That outstanding performance, combined with Riverstone’s stunning natural aesthetics, have seen it accepted for use on historic buildings all over the world.

In Britain, that includes landmarks with Grade II* and Grade I-listed buildings – Christ’s College in Brecon, Powys, the St Thomas Church in Exeter, Kresson Kernow Records Office in Redruth, and Devon’s Grade I-listed Pig at Combe Restaurant and Hotel all have Riverstone roofs.

View Riverstone on roofs in our Riverstone case studies.

If you’ve got a project that would benefit from one of the best natural roofing solutions the world has to offer, please contact us here.