Thanks for replying to me.
I stay in South Africa, in Cape-Town precisely.
So I found them on the beach of Cape Town.
No, the black one's not Magnetic, but when I try to scratch it against a white fabric, it leaves a very few black mark.
I tried to scratch the black one with a knife but I found nothing, I mean it only leaves black scratch mark; and it's tough if you try to scratch it but it's not really heavy.