It's one of the many unofficial Weimar-era decorations:
This is the Mackensen Ehrenkreuz 1. Klasse [Mackensen Honor Cross 1st Class] awarded by the Waffenring der Deutschen Kavallerie. Founded in 1928 under the patronage of Generalfeldmarschall August von Mackensen whose portrait is seen on this cross, this was a blanket organisation that by 1933 included 572 cavalry veterans' associations totaling some 53,500 members.
The decoration was instituted in 1928 and came in a gold and silver class. In 1934, it was replaced by a newly-designed clasp. The cross was awarded to members of the Waffenring der Deutschen Kavallerie, with award criteria unknown.
For some German language information, see:
Mackensen Ehrenkreuz 1.Klasse
Mackensen Ehrenkreuz 2.Klasse
Mackensen Ehrenschnalle 1. Klasse Gold 1934
Mackensen Ehrenschnalle 2. Klasse Silber 1934
Thank you for your reply. I found other decorations like this one on the web, it seems to be a good one.
Do you know its value? We can find about everything... It was bought for €13.