Moon-letters are a system of writing invented by Tolkien and attributed to Dwarfs. Moon-letters do look like ordinary runes, but are written in such a way that they can only be read when the light of the moon shines behind the writing. Some moon-letters can only be read by moonlight, but it is possible to make the writing even more secret. More sophisticated moon-letters can only be read by the same shape of moon, or at the same time of year, as when they were written.
Moon-letters are written with special silver pens. The substance Ithildin, made from Mithril, reflected only starlight and moonlight, and it seems likely that Ithildin, or something very similar, is used by the Dwarves to write their moon-letters.
Moon-letters can be seen on the map of the Lonely Mountain and surrounding areas that Gandalf gave Thorin Oakenshield . The moon-letters on the map are of the more secret type mentioned above, and they were written on a midsummers eve (20 June) in a crescent moon.
'Moon-letters are rune-letters, but you cannot see them.' said Elrond, 'not when you look straight at them.They can only be seen when the moon shines behind them, and what is more, with the most cunning sort it must be a moon of the same shape and season as the day when they were written. The dwarves invented them and wrote them with silver pens, as your friends could tell you. These must have been written on a midsummer's eve in a crescent moon, a long while ago.' - ( The Hobbit: A Short Rest)