In the books
In the A Song of Ice and Fire novels, the Seastone Chair is located on a dais, in the Great Hall of the Great Keep on Pyke. It is made of a block of oily black stone carved into the shape of a kraken. The legend says that it was found by the First Men on the shores of Old Wyk when they came to the Iron Islands.
Although the term "Seastone Chair" is used in the books, the TV series generally prefers "Salt Throne", particularly when used as metonymically to refer to the kingship of the Iron Islands.