It isn't the surface material of the slab you need to change it is the building material.
Check the building material and I think you will find you are using marble.
If you want a uniform cut material rather than the element's own material then you can go into the 'Filter and Cut Elements in 3D' (CTRL+ALT+A on Windows) and you can set a custom cut surface.
Barry.
One of the forum moderators.
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