Nov 27, 2025
Week 10 – Stone, Geology and the Material Intelligence of Landscapes
Every stone carries a history older than architecture - landscapes are shaped not only by design, but by the deep time written into their materials.
Technical
Understanding stone from the ground up - deep time, formation and material behaviour.
This week we were joined by Lisa Nunn and David Richardson from FMDC, both trained geologists and specialist stone consultants. Their practice focuses on facades, paving systems and material specification and they are members of The Stone Foundation - meaning their technical knowledge spans both the geological and architectural worlds.
We began with the three major rock groups, explored in detail:
1. Igneous Rocks
Formed from molten magma or lava cooling and solidifying.
Intrusive igneous rocks (e.g. Granite, Diorite):
Cool slowly beneath the surface, creating large interlocking crystals.
Extremely durable, resistant to weathering, high compressive strength.
Ideal for high-wear paving and load-bearing surfaces.
Extrusive igneous rocks (e.g. Basalt):
Cool quickly at or near the surface.
Fine-grained, dense, often used where slip-resistance and strength matter.
Key behaviour:
High structural integrity
Excellent long-term performance
Good thermal properties but may polish under heavy pedestrian use depending on finish
2. Sedimentary Rocks
Formed through the compaction and cementation of sediments.
Sandstones: quartz-rich, variable porosity, excellent grip when textured.
Limestones: composed of calcium carbonate, often fossil-rich; beautiful but vulnerable to acidic environments.
Shales and siltstones: layered, weaker, suitable for cladding more than paving.
Key behaviour:
Layering affects strength and durability
Susceptible to moisture absorption depending on porosity
Can weather unevenly if bedding planes are incorrectly oriented
3. Metamorphic Rocks
Formed when existing rocks are transformed by heat and pressure.
Slate: fine-grained, naturally cleft, excellent slip resistance.
Marble: recrystallised limestone, visually rich but less suitable for public realm paving due to polishing.
Gneiss: incredibly strong, banded, similar performance to granite.
Key behaviour:
Structural stability and low porosity
Elegant finishes but must be matched to context
Some metamorphic stones lose roughness under foot traffic
Lisa and David emphasised that understanding geological formation is essential to predicting how stone behaves once installed - including weathering, slip resistance, colour variation and structural loading capacity.
Finishes, textures and the science of slip-resistant design.
We moved into the world of stone finishes, exploring how different treatments affect aesthetics, safety, porosity and long-term performance.
They explained how finishes are produced:
Common Finishes
Flamed: intense heat causes micro-spalling → rough, grippy surface.
Honed: smooth, matte, non-reflective → elegant but can be slippery when wet.
Polished: mirror-like → beautiful indoors, unsuitable outdoors.
Bush-hammered: textured through point impacts → excellent slip resistance.
Sandblasted: fine abrasions give softer grip and diffused appearance.
Split-faced / Natural cleft: utilises natural fracture planes → unique patterns with inherent slip-resistance.
We discussed how each stone responds differently to each finish. For example:
Granite maintains roughness well under flamed or bush-hammered finishes.
Limestone can degrade more quickly when textured due to its softer mineral composition.
Sandstone performs well when cleft or lightly textured but may polish under vehicular load.
Safety considerations were a major focus:
Slip resistance is measured through pendulum testing or R-value classifications.
Urban sites require careful selection based on gradient, expected footfall and water flow behaviour.
Some finishes degrade faster in freeze–thaw conditions, altering slip resistance over time.
They also discussed colour consistency, using Trafalgar Square as an example:
Stone from different quarry beds can exhibit subtle but noticeable tone variations.
Over long installations, consistency must be controlled through batch sampling and precise quarry selection.
Sustainability, sourcing and the environmental profile of natural stone.
The session concluded with a deep dive into the sustainability dimensions of stone, which challenge many misconceptions. Lisa and David argued that when sourced responsibly, stone is one of the most sustainable building materials available.
Key sustainability considerations:
Energy & Carbon
Quarrying uses significantly less embodied carbon than producing concrete, brick, or ceramics.
Renewables increasingly power quarry sites.
Many quarries recycle water used for cutting and cooling saws.
Stone often requires minimal processing → low manufacturing emissions.
Ethical Sourcing
Avoiding quarries with labour exploitation.
Ensuring compliance with modern slavery legislation.
Transparent chains of custody through certification schemes.
Longevity & Adaptability
Stone’s lifespan can exceed hundreds of years.
Can be re-used, re-cut and re-polished for extensions or new projects.
Minimal long-term maintenance compared to concrete equivalents.
Embodied Carbon & EPDs
We ended with a discussion of EPDs (Environmental Product Declarations) - third-party verified documents that summarise the environmental impacts of a material throughout its lifecycle.
An EPD evaluates:
Raw material extraction
Energy use
Water consumption
Transport emissions
Production waste
Reuse and recycling potential at end-of-life
This allows landscape architects to compare materials transparently and make informed choices that consider both performance requirements and carbon implications.
Reflection: This session revealed that stone is not merely a “material choice” but a geological narrative, a sustainability argument and a technical calculation. Understanding formation, finish, durability and environmental impact clarified just how much intelligence lies beneath every slab we specify.