Gruenkern is produced in a relatively small area in Southern Germany around the town of Boxberg. In short: Gruenkern is roasted spelt and there are lots of fantastic recipes. Carpe diem!
(Re)discover something special!
Some 800 years ago Hildegard von Bingen, the abbess of an ancient monastry wrote about spelt, the substance from which gruenkern is made: "The spelt is the best of grains. It is rich and nourishing and milder than other grain. It produces a strong body and healthy blood to those who eat it and it makes the spirit of man light and cheerful. If someone is ill boil some spelt, mix it with egg and this will heal him like a fine ointment."
The ears of spelt (triticum spelta) consist of only two rows of grain and it grows best on poor soil. Where no other grain will prosper spelt will do fine.
But spelt isn't gruenkern!
If you wish to make gruenkern from spelt you have to harvest it while the grain (Korn/Kern) ist still green(gruen). So a literal translation of gruenkern would be something like greengrain. Sounds good, doesn't it?
But how did people get the idea to harvest spelt in a green condition and roast it? That's an interesting (hi)story!
Informative and relatively extensive link in English