Cobblestone is longer to break than stone!
If you try with no difficulty to know , read:
1-You need to be in survival mode;
2-Make a wooden pickaxe or turn the cheats on and get one;
3-Break 2 stones and get the cobblestone;
4-Place the cobblestone next to Stone
5-Break the stone , then cobblestone and see the difference
I dont know if is a bug , but stone and cobble had the same explosion resistence (30) and you gain cobblestone by breaking stone!