When I play Minecraft, the icon in the Unity Launcher for it is always a grey question mark. This has persisted from Minecraft 1.6.2 to 1.7.4.
See this screenshot:

How can this be fixed?
I have tried to create a .desktop file to launch the Minecraft launcher, however this only gives the launcher an icon and the game still shows as a grey question mark.
The .desktop file attempted was (adapted from this asnwer):
[Desktop Entry]
Name=Minecraft TEST
Comment=
Exec=java -jar /opt/Minecraft/Minecraft_launcher.jar
Icon=/usr/share/icons/Minecraft_Block.svg
Terminal=false
Type=Application
StartupNotify=true
I've also tried creating a file with gnome-desktop-item-edit --create-new however that doesn't work either.
With either method, if I specify an icon in the .desktop file, the icon is given to the Minecraft launcher, not the game (I want the game to have an icon as well).
I was using Ubuntu 13.04; I've since upgraded (via a fresh install) to 13.10 with the same issue persisting.