/usr/share/gwyddion/glmaterials/Brass is in gwyddion-common 2.44-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | Gwyddion resource GwyGLMaterial
0.329412 0.223529 0.027451 1.0
0.780392 0.568627 0.113725 1.0
0.992157 0.941176 0.807843 1.0
0.0 0.0 0.0 1.0
0.21794872
|