Wasabi

Factoid of the day comes from FXCuisine:

A oroshigane, the traditional wasabi grater made from wood and shark skin.
That's right. Wasabi is not the crap you eat with sushi. That one is an industrial paste made from horseradish and green food coloring.
Real wasabi has a mellower taste and its cost reminds you of French truffles. It is a big greenish phallic affair that only keeps for a few days wrapped in a wet towel in the fridge.
Serious Japanese restaurants (not only sushi places) sometimes grate it at the table, so you know that it is the real deal.
And for this they use a sharkskin grater as fine as sandpaper

more about the oroshigane at Wikipedia. photo here.

A discussion of the various graters and prices here.

The importance of correct grating can’t be overstated.  The prized flavour and

incredible aroma of wasabi is created by ‘tearing apart’ the individual wasabi cells

and mixing the ingredients within. This sets off a chemical reaction and after giving

the paste a few moments to rest and develop its flavour, WOW!  To achieve just

the right texture, a very fine grater is needed.  Utensils such as cheese or nutmeg

graters just don’t do the job properly as they are too coarse and ‘slice’ rather than

‘grind and mix’.

so why is wasabi so expensive?
Because the best roots are grown in stream.  Dave's Garden has photos and links to this video: