Copper Antique 300+year old need help

Hello my fellow steemit friends i need help from you . I got this Antique which you can see in the pictures it was found by my grandfather from the lake near our house ,while he was fishing he saw something shining at the deep part he dived n got it. It was around 12years ago ..now what i want to say is
How can i know its age ?
How much it costs ?
How can i get more informations about it