Pistachio trees are native to the Middle East and have been around for centuries, dating back to 6000 BC. It's been said that Queen Cleopatra declared pistachios a royal food and hoarded all of the pistachios during her reign.
Pistachios are unique to other nuts because the shell splits open when they ripen as the nut outgrows the shell. This is why it is said to look like it is smiling. The Middle Eastern people refer to it as the "smiling pistachio" and the Chinese call it the "happy nut."
Pistachios make the body happy too as they have many nutritional benefits. Pistachios are cholesterol free and high in fiber and folate. They are also the nut with the highest number of antioxidants, even more than green tea according to the USDA scorecard. The two antioxidants in pistachios that are "sight saving" are lutein and zeaxanthin.
Pistachio nuts are naturally green in color. Green is often associated with health, hope, and renewal and alleviates anxiety.
So there you go...get some happy nuts.
Sources: Americanpistachios.org, Michigan State University Extension, Bourncreative.com-Meaning-of-the-color-green.