COBBLESTONE:
package me.drbooker.breakevent.listeners;

import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.BlockBreakEvent;
import org.bukkit.inventory.ItemStack;

public class BreakEventCobblestone implements Listener {

    ItemStack cobble = new ItemStack(Material.COBBLESTONE, 1);

    @EventHandler
    public void onBreak(BlockBreakEvent event) {
        Player p = event.getPlayer();
        final Block blok = event.getBlock();
        if(blok.getType() == Material.COBBLESTONE) {
            if(p.getItemInHand() != null && p.getItemInHand().getType().toString().contains("PICKAXE")) {
                p.getInventory().addItem(cobble);
                event.getBlock().getDrops().clear();
            }
        }
    }
}

STONE:package me.drbooker.breakevent.listeners;

import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.enchantments.Enchantment;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.BlockBreakEvent;
import org.bukkit.inventory.ItemStack;

public class BreakEventStone implements Listener {

    ItemStack stone = new ItemStack(Material.STONE, 1);
    ItemStack cobble = new ItemStack(Material.COBBLESTONE, 1);

    @EventHandler
    public void onBreak(BlockBreakEvent event) {
        Player p = event.getPlayer();
        final Block blok = event.getBlock();
        if(blok.getType() == Material.COBBLESTONE) {
            if(p.getItemInHand() != null || p.getItemInHand().getType().toString().contains("PICKAXE") && p.getItemInHand().containsEnchantment(Enchantment.SILK_TOUCH)) {
                p.getInventory().addItem(stone);
                event.getBlock().getDrops().clear();
            }
            if(p.getItemInHand() != null || p.getItemInHand().getType().toString().contains("PICKAXE")) {
                p.getInventory().addItem(cobble);
                event.getBlock().getDrops().clear();
            }
        }
    }
}

MAIN:
package me.drbooker.breakevent;

import org.bukkit.Bukkit;
import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

import me.drbooker.breakevent.listeners.BreakEventCobblestone;
import me.drbooker.breakevent.listeners.BreakEventStone;

public class Main extends JavaPlugin {

    public static Plugin pl;

    @Override
    public void onEnable() {
        pl = this;
        System.out.println("BookBreakEvent");
        System.out.println("Author: Booker");
        registerEvents();
    }

    @Override
    public void onDisable() {
        pl = null;
        System.out.println("BookBreakEvent");
        System.out.println("Author: Booker");
    }

    public static Plugin getInst() {
        return pl;
    }

    private void registerEvents() {
        PluginManager pm = Bukkit.getPluginManager();
        pm.registerEvents(new BreakEventCobblestone(), this);
        pm.registerEvents(new BreakEventStone(), this);
    }
}