/usr/share/gmt/custom/diamond.def is in gmt-common 5.3.1+dfsg-2.
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 7 8 9 10 | # $Id: diamond.def 12822 2014-01-31 23:39:56Z remko $
#
# Definition file for diamond symbol. Use in psxy
# as -Skdiamond/size. Unlike -Sd, this allows for
# using patterns as fill
#
0 0.5 M
0.5 0 D
0 -0.5 D
-0.5 0 D
|