Philip II 8 Reales Coin? Fake?

The coin resembles an early issue from Peru, very much like this one

It has the appearance of a coin salvaged from a shipwreck. Do you have a certificate of authenticity from a salvage company? If it is made from coin silver, it will be difficult to authenticate without a COA. On the other hand, if it's not silver then you definitely have a replica.

As for "cob" coins, the Spanish word is macuquina and a better explanation of how they were made can be found here.

18.6 grams is way too lightweight to be genuine. A shipwreck coin will lose some weight in the cleaning process, but not that much. To get that much of a weight reduction, you'd expect to see some pieces physically missing.

From the CCF discussion at http://goccf.com/t/344533 please keep in mind that a silver /copper alloy coin confined to "Davy Jones Locker" i.e. bottom of the ocean, for many hundreds of years can lose the copper rich (alpha phase) grains through electrolysis and then end up as a "spongy" coin composed of silver rich (beta phase) grains. This would also reduce the coins weight and specific gravity.

Squire