This is a bit off-topic (not related to Buddhism per se), but as I was investigating the properties of green tea, I found an excellent article about it from the University of Maryland Medical Center:
Green, black, and oolong tea are all derived from the leaves of the Camellia sinensis plant. Originally cultivated in East Asia, this plant grows as large as a shrub or tree. Today, Camellia sinensis grows throughout Asia and parts of the Middle East and Africa.
People in Asian countries more commonly consume green and oolong tea, while black tea is most popular in the United States. Green tea is prepared from unfermented leaves, the leaves of oolong tea are partially fermented, and black tea is fully fermented. The more the leaves are fermented, the lower the polyphenol content [this is the antioxidants that provide health benefits], and the higher the caffeine content. Green tea has the highest polyphenol content, while black tea has roughly two to three times the caffeine content of green tea.
Note that we Western people prefer the tea that has the highest caffeine content and the lowest polyphenol content! This contradicts some information I had previously heard about the amount of caffeine in green tea, relative to coffee. This lower-caffeine claim was backed up by this chart showing relative amounts.