Wow! Take it to a jeweler and let us know what he says!
I have seen pictures of some of the older Spanish pieces with pearls, (look in the older Nat'l Geographic shipwreck issues) and usually the pearls are dissolved over time, so this piece may not be that old. Pearls are soft and may be ground down by shifting sand. Another indication of age is how the stone was cut. Diamonds are very hard and only within the last 50 years has the modern 'Brilliant' cut been perfected. A jeweler could tell you whether the stone is genuine, and might have some idea of when it was cut based on its form.
You asked when they put karat markings on gold-- that tradition was started by the Spanish for bullion bars at the assayer, who used Roman numerals to denote karat purity. This wasn't applied to jewelry until much later (late 1800's), when standard alloys began to replace pure gold which is very soft. That doesn't mean that karat markings always appear on gold-- even these days of mass production a few pieces make it out of the factory unstamped.