Here's a blurb I found on a website devoted to saffron. It is an essential in lots of cuisines around the world.
Saffron is the world's most expensive spice. And that is because it uses only the three strands of the crocus sativus flower - a type of iris, which blossoms in the autumn in the northern hemisphere. At the time of harvest the three red strands or stigma need to be plucked from each flower by hand. This is time consuming and labour intensive and means that about 500,000 strands will need to be gathered in this way to produce 1kg (2lbs) of the spice.