They are just pebbles that happen to be made of a mixture of different rocks. Such mixtures can be composed of rounded or angled fragments in a matrix of other rock. That can happen with sedimentary or igneous rocks, and on various scales.
It can also happen in fault zones - where the rock gets fractured and gaps are left between the pieces which get filled with minerals precipitated from ground water. I say 'ground water', but actually, that can be a hot and pressured 'soup' of dissolved minerals. I am fairly sure your specimens are of this origin - pieces of host rock broken during faulting and later 'glued' back together by what appears to be quartz.