Comes without the string so you can choose your own.
Epinephrine (also known as adrenaline or adrenalin) is a hormone and a neurotransmitter. Epinephrine has many functions in the body, regulating heart rate, blood vessel and air passage diameters, and metabolic shifts; epinephrine release is a crucial component of the fight-or-flight response of the sympathetic nervous system. (from Wikipedia)
This model is rather big. You can find a smaller, more suitable for earrings, keychains or pendants here: http://shpws.me/qHAS