User blog comment:Somarinoa/New Pages to Expect - January 2014/@comment-160529-20140105182322

grey alien would be best for aliens, a silhoutted earth (continents cut out) for worlds (not just planets, but perhaps a same for luna if you wanna differentiate?)

substances you could probably go with a periodic table marking in front of a vague rock, with the values of the periodic table tile being question marks instead of letters and numbers... but then again sometimes thats all we know about vague substances.