The Lizard Detection Stone is an item gained through beating 8 colossi on Normal Time Attack, or 10 on Hard Time Attack. It is a small golden statue of the lizards that inhabit the Forbidden Lands.

Dormin says the following upon retrieving the item:

That is the "Lizard Detection Stone"...
By using it, thou shalt be able to find the lizards throughout the world...

This stone shows the location of the white tailed lizards in immediate area around you. They show up on your map, but the lizard's elevation will not be shown. You have to search carefully for the lizard, because it could be right behind a wall or hiding in some bushes. The symbol disappears when the lizard is killed, but not when the tail is pulled off by shooting at only the tail. Therefore, although more merciful, it is not a good idea to shoot the tail off of most lizards.

Like the Fruit Tree Map it isn't an item that is required to carry, rather equipped and activated on the map.