Use "on". Something doesn't have to "stand on the surface" of something to be "on" it. It simply has to appear on the surface.
I have scars on my back.
There are spots on his skin.
There are marks on the lamp.
I can see some strange dark areas on the Moon.
If you told me that there was a decoration or a mark "in the lamp", I would try to look right inside it, ie I would remove the lid and try to peer into the interior of the lamp.