/usr/share/gmt/custom/vneedle.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 | # $Id: vneedle.def 12822 2014-01-31 23:39:56Z remko $
#
# Definition file a vertical very narrow diamond-like symbol.
#
0.1 0 M
0 0.5 D
-0.1 0 D
0 -0.5 D
|