/usr/share/gem/examples/data/venus.mtl is in gem-doc 1:0.93.3-7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# venus.mtl
#
newmtl vskin
Ka 0.6 0.6 0.45
Kd 0.6 0.6 0.45
Ks 0.2 0.1 0.1
Ns 5.0
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