Here's a non-technical article re: quality issues with olive oil

http://www.themonthly.com/feature08-05.html

The lead-in for the article says:
Cooks who use oil that's labeled as "extra virgin" might be dismayed to
learn that some brands blend in hazelnut oil and then dye the mixture with
chlorophyll. Legitimate olive oil producers say "like a virgin" just isn't
good enough. They want to see federal guidelines in place.

Tom Billings