I did chem in high school, so should know... from what I remember of the top of my head, without looking at my old books, it is rarer then gold, and technically more valuable, but as to it "over taking gold", while it might make sense to have platinum medals at the Olympics, as it is more valuable, it is far to rare. And yes, it was known of in past centuries. It was used to some extent by South American Indians, but only really became known to the world after the Spanish conquests (so about 16th or 17th centuries).
Largest deposits I think are in Siberia (Russia) and Africa, but even there it is very rare.
And its a naturally occuring metal, so its probably been around for millions of years at least, its only been known about for a few hundred.
It's a metallic substance that does not naturally grow/ is made on earth, the large deposits were from comets, meteors and other junk flying around in space. if i remember rightly it' works the same way as diamonds, put enough pressure onto ash (human, dog anything with carbon) and it will form into a diamond with some scientific magic... though the process cost more then the diamoind is worth. Platinum as far as i know is another material compressed over billions of years in space, forming asteroids...
platinum like gold is a chemical element number 78 symbol Pt (Gold is Au number 79). Because of it qualities being rather similar to gold and the fact that it also doesn't tarnish, it is very useful in making conductive wire and is used as a catalyst in some chemical reactions.
The reason why it's valuable it largely due to the fact that it's rare and that people want it. Technetium is far rarer but no-one wants it. Two things will determine the value of something - how much of it there is, and how many people want it.
Read yourself silly:
http://en.wikipedia.org/wiki/Platinum