I just downloaded blender 2.8 source in order to create own tool icons. Before done that I googled up and been done at blender wiki
If you will scroll down you will see such line:
Tools in the toolbar have to supply an icon. Here's how: In blender 2.8 we have the possibility of creating binary icon files (.dat) from blender files (.blend). You can see some examples of these blender files with icons in the "icon_geom.blend" in the library with the blender dependencies. (..lib/resources/icon_geom.blend).
So I tried to find that …lib/resources/icon_geom.blend but there’s no such file therefore it’s really hard to come up with how to make and tool icon for blender.
And file which was on developer blender probably got deleted. Icon_geom.blend