You can tell the rarity of an item based on its color. Gray items are common, blue items are rare, red items are ultra-rare, and gold items are legendary, which means they can only be obtained from special events. Dragonstones are common, but as Mirkrilaar mentioned, there are quite a few of them, so getting a specific dragonstone is by chance.

It is actually more common than you might think. There are 35 or so dragonstones and they are regarded as "rare" items, but they are very common to get due to their numbers. Although specific ones are by chance.

Check my hoard to see the rarity of any items!

*muttering* Showoff...

It's not show off, it's just that my hoard is a reference, it's helping, you my friend, have shown off (never)

No, not really

BoDuSil

No, not really 

Is that seriously your answer?

Eh, I'd say not really.