My fav sushi restaurant serves this leaf with their seaweed salad. The flavor is beyond description - it is more like a journey than a taste! The only name they have for it is 'mint'. I know what you are thinking, but their supplier only ships the leaves - no stems to check for the square shape.
I think I may have figured it out, but its only a guess and wanted to see if anyone knows for sure what it is. It is a bit rumpled and has darkened more than it was before its journey home in my purse :-0

Gosh that came out BIG! Its usual size is about 1 1/2 by 1 inch. And I must say it was of great sacrifice to save it for a pic, instead of enjoying it with the salad!

More like a journey than a taste, sounds like cannabis to me Maggie. :-)

I wouldn't know, how would you?
Speaking of which :) the movie 'Saving Grace' is out on video - bout a Brit gardener with a questionable crop. Maybe it was the sushi herb.)
You appear to have the qualifications for newspaper editing Terry. The flavor grows and changes with every chew, very exotic.

Hey! I could get into trouble, canary seed contains hemp, wonder where I would stand legally if some of that germinated? Newspaper editor with my atrocious spelling hmmmm. maybe you are right Maggie. J

What with my story-writing style, I'd make a lousy reporter. ;-) and that's just fine with me!

Now look what you've started, Mr. Greenjeans... it's Maggie's Story Time ....
The town of Glastonbury in Somerset is reputed to be the legendary site of King Arthur's realm and is widely visited by tourists, mystics, spiritualists and users of all sorts - think of a 'New Age' version of the old Haight Ashbury. Anyway, walking down the sidewalk, I notice this window with a mountain of seed in it. What gardener wouldn't put on the brakes and go in? Bought a great hat there. Turns out, it was of woven hemp and the seeds were Maryjane. Apparently, the proprietor is an advocate of legalizing weed and that his mountain of seed was legal over there,,, and I think he said mj was being prescribed legally, for 'medicinal' use at that time. Now I may have gotten that all wrong, and if so, I'll blame it on my narrator's overly medicated state.

Okay, here's what I think the mystery herb is...
a Perrilla. The back one that re-seeds too much in our gardens has black leaves and smells like licorice, where this green one taste wonderful. I found one ref to a green perilla being used as a salad herb in Japan, and this may be it. I would love to grow my own kitchen supply of this tasty thing.