[Making a glass block]

I am trying to get a glass block with a dirt texture, but I can't figure out what it is wanting in place of the "null"

 

It's asking for a string, and I am at a loss.

 

package com.johnrad.Block;

import com.johnrad.lib.RefStrings;

import net.minecraft.block.Block;
import net.minecraft.block.BlockBreakable;
import net.minecraft.block.material.Material;
import net.minecraft.creativetab.CreativeTabs;
import cpw.mods.fml.common.registry.GameRegistry;

public class ModBlocks {
public static void mainRegistry(){
	inistializeBlock();
	registerItem();
}

public static Block ComStone;
public static BlockBreakable DirtGlass;


public static void inisializeBlockBreakable(){
	DirtGlass = new BlockBreakable(null, Material.glass, false);
}
public static void inistializeBlock(){
	ComStone = new ComStone(Material.ground).setBlockName("ComStone").setCreativeTab(CreativeTabs.tabBlock).setBlockTextureName(RefStrings.MODID + ":ComStone");
}


public static void registerItem(){
	GameRegistry.registerBlock(ComStone, ComStone.getUnlocalizedName());
}
}

 

Any help please?