ずいぶんと珍しいですね。の英語

It's so rare.

That is rare.

This is a very rare specimen.

Unusual, what?

the degree of novelty of something

That's unusual.

the quality of being novel

a thing of little curiosity

something that is very rare

This is a little weird.

You don't say.

That's a surprise.

something being very uncommon

Very peculiar

the state of something being novel in some way

That's strange.

How strange!

the degree to which something is so rare there are no other examples with which to compare it

That was unexpected.

That's surprising.

That's a surprise.

Uh, now it's really weird...

That is very strange.

That is very unusual.

That is strange.

oddly [curiously, strangely] enough

with curiosity

It is truly wonderful.

It is a curiosity―a rarity―a rara avis.

I am surprisingly alright.