This is an example from one of my crafting recipes. Declare the item/block first
ItemStack purpleIngot = new ItemStack(ModItems.purpleIngot);
ItemStack stick = new ItemStack(Items.stick);
GameRegistry.addRecipe(new ItemStack(ModItems.purpleAxe), "xx ", "xy ", " y ",
'x', purpleIngot, 'y', stick);
Update:
Getting obsidian for a recipe.
Should be functional for your recipe.
ItemStack obsidian = new ItemStack(Blocks.obsidian);
ItemStack redDiamondGem = new ItemStack(ModItems.RedDiamondGem);
ItemStack redStoneDust = new ItemStack(Items.redstone);
GameRegistry.addRecipe(new ItemStack((ModItems.RedDiamondGem), "ror", "rdr", "ror",
'r', redStoneDust, 'd', redDiamondGem, 'o', obsidian));