It's just an apparition of the mind and nothing particularly important.  Things like this will arise from time to time.  Like Max said in the comment, the mind subconsciously tries to entertain itself.  This actually happens, like clockwork, at set times in a sit the first usually occurring right around the the 20-25 minute mark.  This is where we really begin to shift from ordinary discursive consciousness to something resembling a meditative state.  Other phenomenon you might experience are feelings of your body twisting, your head or hands inflating, your muscles shaking involuntarily, your face disappearing, or, like you mention, your body feeling like it's made of stone.  FWIW, I would actually say that the feeling of being made of stone is more indicative of progress than the light.  If you can sit unmoving for even longer, a similar shift will happen at around the hour and fifteen minute mark when the body/mind's natural energy begins to arise.  This is where things get interesting.

You're right at the cusp of passing beyond the beginner stage.  If you can find a way to commit yourself a little more, you're practice will start to have a real impact on how you live your life and experience the world.  My advice to you is to sit as stone for as long as you can.  Try to push your sits towards the hour mark and see what happens.