When it comes to collector coins, many "practical" considerations can be left aside. Coins that we use in everyday life are usually round, flat and made from some metal or alloy that is not too expensive. But a piece that collectors will pay more than face value for, and that will not be used in circulation, can be different. You may remember the "Ultime Franc", designed by Philippe Starck and issued in 2001, which had a "wave" shape. This year, the French mint made another piece that is not flat.
To commemorate the International Year of Astronomy 2009, and the 40th anniversary of the first Moon Landing, the Monnaie de Paris issued several coins with basically the same design. However, the €10 silver piece is what the mint calls a "domed" coin, with the moon relief on a convex reverse. Yes, another gimmick, and one that costs roughly 40 to 50 euro. But at least the theme is interesting (many countries have issued coins on either occasion or both), the design (plus the shape) is attractive, I think. Here are some photos that jore2665, a French collector, made:



Just for the record: The other silver coin (€50) has a "flat" shape as far as I know, and so does the €50 gold piece. These cost between 350 and 400 euro. And then there is a €200 gold coin, with that convex reverse again, and the stars on a blue background. Will cost you about 1,500 euro.
Christian