what is wasabi made of?

Wasabi is made from ground-up Japanese horseradish. The green coloring is an added dye.

From Japanese horseradish.

Most of the "wasabi" you get in the USA is made from horseradish and sometimes mustard with food coloring, as Sara says. However, true Wasabi is a type of root, similar to horseradish, that is naturally green. It is very expensive and until recently wasn't widely available outside of Japan. Here's more on real Wasabi: http://www.foodista.com/food/FCM5KTGZ/wasabi

True Wasabi is made from the vegetable, Wasabia japonica. As Sara and Barnaby say the "wasabi" available in shops is coloured horseradish and mustard mixtures. Sometimes with lots of nasties added for good measure. Have a look at www.wasabireviews.com to see what is in some of these green goo mixtures. Also there is a lot of articles, recipes and information on True Wasabi to be found at wasabi.org. Wasabi.org has started a campaign to get stop coloured horseradish being called Wasabi! You can join the cause here http://www.causes.com/causes/594403-stop-coloured-horseradish-being-called-wasabi

Wasabi from a store or a market is actually just horseradish died green because wasabi is very protected and only grown in one region of Japan making it very expensive