A cut face is a different material

Breaking something exposes an interior. In a field that interior costs nothing to produce, which is the appeal, but it arrives with no material of its own. Give it the shell's material and the pieces stop reading as broken.

A hollow neon ring splitting. Left: the cut faces shaded like the outside, so each half grows its own rim. Right: the exposed wall cross-section treated as freshly cut material.

On the left each half looks like a smaller intact object, because every surface it has is shaded as an outer surface. On the right the flat face reads as a fracture: hotter, denser, with its own bright edge where the break happened.

Knowing which surface you are on

The useful signal is which term of the intersection is active. A piece is max(body, cutPlane), so at any pixel one of those two is the one producing the distance: if it is body, you are on the original surface; if it is cutPlane, you are on a face that did not exist a moment ago.

float piece = max(body, cutPlane);
float onCut = smoothstep(bandWidth, 0.0, abs(cutPlane));
vec3 mat = mix(shellMaterial, interiorMaterial, onCut);

Blend rather than switch. A hard boundary between two materials along the cut looks like a decal, and the eye reads a soft transition as the shell having thickness.

Why a shell makes this matter and a solid does not

Worth being clear about when this is worth any effort. If the object is a flat fill, the interior is the same color as the exterior and there is nothing to reveal. This only pays when the outside is structured: a neon shell with a bright rim and a dark middle, a hatched surface, a gradient. Then the flat cut is genuinely new information and its absence is conspicuous.

Which is also the ordering advice: decide what the object looks like whole, and only then decide what it looks like broken. A cut material designed first tends to look unrelated to the thing it came out of.

The cheap details that carry it

A bright line exactly on the cut. A fresh break catches light along its edge, and this does more than the face color does.
Fade the interior in as the pieces separate. At zero separation there is no exposed face, so it should not be visible yet.
Make it hotter, not just different. Interiors read as denser and more energetic than surfaces, which is why a warm interior under a cool shell works so reliably.

Rules of thumb

Which term of the max is active tells you which surface you are on. Use it.
Blend across the boundary. A hard switch reads as a decal.
Only worth it when the outside is structured. A solid fill has no interior to reveal.
Design the whole object first, then the break.
Fade the interior in with the separation, and put a bright line on the cut itself.

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