This rock is smooth, heavy and dark grey/black in color with white blotches which are different in shape and size and are also porous. It scratches by a knife and I found it in with many pails of rocks when I moved into my place. The owners prior to me bought it in the 60's. The house was built in 1910. Some of the rocks in the pails are valuable. This one I cannot find to identify.[
Your rock appears to be some kind of breccia. The first photo shows a small round shape near the middle that might be a crinoid stem section. My best guess is that the rock is limestone or dolomite breccia. Acid test with HCl should confirm if it is limestone.