How is mushroom stone made?
Mushroom stone is not carved, it is split. Understand that and you can see why its thickness tolerance is far wider than a polished slab, and where to look when inspecting it.
What does it look like, and why the name?
The face rises in the middle and falls away at the four edges, like a mushroom sitting on the slab. The raised part is a natural split face — rough, undulating, showing the raw character of the stone. The four edges are flat, for bedding mortar or overlapping. It is used for plinths, boundary walls and feature walls precisely for that coarse relief.
How is it actually produced?
Mainly by splitting. The slab is cut to size, then a splitter or a hand tool applies pressure along a predetermined line, and the stone fractures along its grain to leave a natural uneven face. The four edges stay flat for installation. Nothing is carved and no mould is used — the random look comes from the fracture itself.
Why is the thickness tolerance so much wider?
Because splitting follows the weak planes inside the stone, and where it breaks is not fully controllable. Within one batch a few millimetres of variation in relief height is normal. So do not order it against polished-slab tolerances; agree a range for relief height and a range for edge thickness. Those two numbers are its real specification.
Does the split face hold dirt and grow moss?
It does, especially in damp or shaded positions. A rough surface holds water, water plus dust becomes grime, and moss follows. The answer is not to polish it — that destroys the whole point — but to apply a water-repellent or sealer before installation, and to accept that it is meant to look a little weathered.
What three things do you check on inspection?
First, whether the four edges are flat, since that is the bearing surface and uneven edges will not bed. Second, whether relief heights fall within one range, because pieces that differ too much will not sit level together. Third, whether there are through-cracks — splitting does not produce them, so their presence means the block had a hidden fissure. The first two are visual; the third needs every piece turned over.
Which stones suit mushroom stone?
Coarse-grained, evenly structured granite suits it best, because the split face then shows clear crystalline texture. Fine-grained stone splits nearly flat and loses the point. Stone with strong bedding, some slates for instance, peels along the bedding plane and will not form. So when choosing the stone, ask for a split sample, not only a polished one.