What does the coin used by the Faceless Men look like?

The coin is described below, from that quote we have the following information:

• It's made of iron
• No larger than a penny in size (penny here referring to a penny in Westeros)
• Rusted along the rim
• One side had writing on it, likely Valyrian (because Valar Morghulis is a Valyrian phrase)
• The other side had a man's head on it but was worn down

Sometimes she wished she had gone off across the narrow sea with Jaqen H'ghar. She still had the stupid coin he'd given her, a piece of iron no larger than a penny and rusted along the rim. One side had writing on it, queer words she could not read. The other showed a man's head, but so worn that all his features had rubbed off. He said it was of great value, but that was probably a lie too, like his name and even his face. That made her so angry that she threw the coin away, but after an hour she got to feeling bad and went and found it again, even though it wasn't worth anything.

^{A Clash of Kings, Arya X}

---

Shire Post Mint are:

Officially licensed by George R. R. Martin, these coins come straight from the pockets of Westerosi kings and commoners.

Therefore, showing what their version of the coin looks like is a good idea. They have it on their website as Iron Coin of the Faceless Man, the description for this coin states:

This is the coin given to Arya Stark by the mysterious assassin Jaqen H'ghar.

^{_{Click image to embiggen.}}

From this image we can clearly see that the writing on the coin is:

Valar morghulis

Valar dohaeris

---

It can be seen clearly in these two images taken from the show as well:

^{_{Click images to embiggen.}}

As a fan of the TV Show, I'd just like to complete @TheLethalCarrot's answer. Seems that not only the TV show coin is different from the description in the books as well as the officially George-R-R-Martin-approved coin, but it also seems that they used at least three different coins in different seasons! Look at the screenshots:

S02E10:

S03E10:

S04E10:

SE502:

5x03:

Seems like we fans take much more care about how the coin looked like than the TV producers :-) Might be a bit disappointing, but that's many times the case. So we should let our fantasy paint the picture :)

PS: The coins from Seasons 4 and 5 are definitely not just iron.

PS: the one from Season 4 was what stuck in my memory, probably as it is the most mysterious one :)