This week we were joined by Lisa Nunn and David Richardson from The Stone Federation, both trained geologists and specialist stone consultants at FMDC. Their work sits between geology and architecture, focusing on façades, paving systems and material specification.

We began with the three major rock groups, examining how their formation directly influences behaviour in landscape applications.

1. Igneous Rocks
Formed through the cooling and solidification of magma or lava.

• Intrusive igneous rocks such as granite and diorite cool slowly beneath the Earth’s surface, producing large interlocking crystals.
  This results in:

  • High compressive strength

  • Low porosity

  • Excellent resistance to weathering

• Extrusive igneous rocks such as basalt cool rapidly at or near the surface, producing fine-grained, dense material.

These properties make igneous stones ideal for:

• High footfall paving

• Vehicular loading

• Structurally demanding environments

However, depending on finish, some may polish under sustained pedestrian movement, reducing slip resistance over time.

2. Sedimentary Rocks
Formed through the deposition, compaction and cementation of sediments.

• Sandstones are typically quartz-rich with variable porosity and perform well in paving when correctly oriented and textured

• Limestones are composed primarily of calcium carbonate and may contain fossils, but are vulnerable to acidic weathering

• Shales and siltstones are layered and relatively weak, often unsuitable for exposed paving

A critical factor is bedding plane orientation. If stone is laid incorrectly, parallel to load rather than perpendicular, it can delaminate or fail prematurely.

3. Metamorphic Rocks
Formed when existing rocks are transformed by heat and pressure.

• Slate is derived from shale through low-grade metamorphism, producing a fine-grained, cleaved structure with strong slip resistance

• Marble forms from recrystallised limestone, giving it visual richness but making it prone to polishing and therefore less suitable for external paving

• Gneiss is a high-grade metamorphic rock formed either from igneous rock (orthogneiss) or sedimentary rock (paragneiss), resulting in banded structures with high strength comparable to granite

Metamorphic rocks typically exhibit:

• Low porosity

• High structural integrity

• Distinct directional properties depending on mineral alignment

Lisa and David emphasised that understanding formation allows designers to predict:

• Weathering behaviour

• Slip resistance over time

• Colour variation

• Load-bearing capacity