Name that Mint

Michele and I had a nice dinner tonight at the Jasmine Garden, a Vietnamese restaurant a few blocks from our flat. In case you've never been, many Vietnamese dishes are served with a side plate of garnishes, including lots of mint and basil. I brought home some of the mint, since I've never seen it before, and I'm hoping that someone out there can tell me what it is.

The waiter said it was a mint they get from Thailand. I didn't want to press him on it, but do they really fly in mint from Thailand? It almost looked like regular mint, except that each bunch had a couple freakishly big leaves. The other odd thing was the leaves are dark green on one side and purple on the other. The taste was minty, with basil-ish undertones. Can anyone help enlighten me?