Things of Middle-earth
Specially-written runes invented by Dwarves that are only visible under the light of the moon:
moon was shining in a broad silver crescent. [Elrond] held up the map and the white light shone through it. 'What is this?' he said. 'There are moon-letters here, beside the plain runes....'
'What are moon-letters?' asked [Bilbo]....
'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 more 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.... These must have been written on a midsummer's eve in a crescent moon, a long while ago.'
The Hobbit, Ch 3, A Short Rest
Elena Tiriel 26Sep05, 13Apr10