>

Right but what's the relevance of including stuff the other side of the galaxy? Cool that it exists and all, but scarcity is for all practical purposes defined by how easy the resource is to actually get hold of (much like oil reserves in utterly inaccessible locations don't count for much)
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Very true. 
There are four "green sand" beaches in the world.  The green comes from olivine (peridot) mixed in with the sand.  Tiny crystals, to be sure, but plentiful. >

Even without considering other planets, diamonds are not all that rare compared to other gemstones. The De Beers company used a number of methods, some rather shady and unsavory, to artificially inflate their value. >

We can also artificially make diamonds. Can't artificially make insects trapped in amber >

I mean…you can… >

Exactly. That's what Tuesday afternoons are for. >

Or perhaps the DNA itself is the true value? Perhaps varied DNA samples are cosmic currency, that would make earth a giant bank….. >

Considering scientists have found over a thousand alternatives to DNA, all carbon-based, maybe it's just life that's valuable. >

The idea that a diverse databank of what are essentially “blueprints” to create a life form is the principle; a  plant or animal may have a relatively short life, but being able to re-create them would be invaluable. You make a good point though, not all life forms may have DNA, whatever building blocks of life exist would be the currency. >

If you had an interstellar trading partner, it would already reduce the rarity of your item. >

Diamonds are just slightly fancy rocks. The only reason they are considered to be as valuable as they are considered to be, is because of marketing by DeBeers, the people who have basically a monopoly on diamonds on this planet. >

One of the reasons we use artificial diamonds in industry nowadays. >

And why everyone should be using artifical diamonds anyway.
They're likely more pure, cheaper, and don't have the blood of child slaves on them. >

I thought you meant just on earth, because I can imagine there are many amber pieces with unique examples of insect species >

I wish I still knew where it was, but back in the 90's during the height of Jurassic Park mania, I bought a piece of amber with a small mosquito in it because, I mean, what kid wouldn't want to have that when you're a huge JP fan?!?
Anyways, I still remember what it cost at the gem store. $30. >

Yes I've always thought that anything durable of biological origin like coral, wood, bone, fossiliferous stone would be valuable in interplanetary trade.  I imagine some  alien potentate impressing their guests with a dinner table top of solid earthwood (assuming these aliens are made of meat).