Because with a universe as big as ours, some things like diamonds are valuable here because they are scarce. But somwhere else.... maybe not so much. Have to love how big of a universe, with so many galaxies and stars, even the rarest of things can be plentiful; hopefully the religous one day can respect statistics in numbers and long durations of time.
Diamonds are actually not all that rare on Earth. The reason they're so expensive is that one company, DeBeers, essentially has a global monopoly on them, and it has successfully foisted a variety of ideas on various societies, such as the notion that an engagement ring has to have a diamond in it, and that diamond has to be a certain size, and so on.
Diamonds are very commonly used in drill bits because of their hardness, and if you do a Google shopping search on diamond drill bits, you'll see that they're not very expensive. Although, admittedly, the diamonds used for drill bits are not gem-quality diamonds.
An interesting article, in any event, though. The planets that are being discovered are a lot stranger than I would have expected -- such as this one having a year that's only eighteen hours long. Very surprising.