Creating a Basic Block

First of all, create a file in your mod's blocks directory and call it stoneCloneBlock.js. Add the following contents to it:

name = "stoneClone";
id = config.getBlockId("stoneCloneId");
displayName[0] = "Stone Clone";
addToCreative[0] = true;
creativeTab = "buildingBlocks";
hardness[0] = 1.5;
resistance[0] = 10.0;

textureFileXP[0] = "/stone.png";
textureFileXN[0] = "/stone.png";
textureFileYP[0] = "/stone.png";
textureFileYN[0] = "/stone.png";
textureFileZP[0] = "/stone.png";
textureFileZN[0] = "/stone.png";

Then, open the mod.js file and add the following lines to it:
config.addBlockIdProperty("stoneCloneId", 600);
mod.addBlock("stoneCloneBlock.js", "normal");

That's it. You find the block in the building blocks tab of the creative inventory.