K&L silver modernist pendant

I have had this pendant for some years now. I also have the original silver necklace that fits the pendant, but I dont like it. Its more like a silver 'collar' and it doesnt close very well. So I'm always afraid I will loose it when I wear it. Thats why I usually wear it on a regular chain.
Its about 5cm long, its 800 silver and its marked K& L which in my opinion stands for Kordes & Lichtenfels, a German company that had its factory in Pfortzheim.
Its from the sixties and as you can see, I never clean my jewelry. I like it that way: nice patina and easy for me ;-)
The center stone could be aventurine or christophase, I dont know. What do you think?
Hope you enjoy it.