I want shiny nails: help!!

A few weeks ago I was in the shoppingcentre where they were hosting a special event.
It was for kids my age and a bit older.
It was too crowded, so I went.

The next day a friend showed me her nails.
Someone had moved a block of something grey over them and now they were very shiny.
Clearly it had been some polish thing.

Now I wonder what that grey thing was.
Who can help me?