Oolong teas, or 'blue' teas, as they are sometimes called, are partially or semi-oxidised/fermented teas. Predominantly produced in China and Taiwan, oolongs are produced from the 'proper' tea plant, the 'Camellia Sinensis'.
Lesser known and more complex in flavour than most black teas, good quality oolong teas are almost on par with fine wines for expense and enjoying sparingly. After all, the most expensive tea in the world, Da Hong Pao, is a rare dark oolong. However, many are an affordable, healthy luxury, with lots of health claims made about their anti-oxidant and weight loss properties!