mc.getRenderItem().renderItemAndEffectIntoGUI(ItemStack, x, y);

This worked for me. Haven't found this one on this forum yet. Maybe it helps somebody.

and if you want to get rid of some shade when rendering blocks

RenderHelper = Java.type("net.minecraft.client.renderer.RenderHelper");

RenderHelper.enableGUIStandardItemLighting()
mc.getRenderItem().renderItemIntoGUI(stack, x, y);
mc.getRenderItem().renderItemOverlays(mc.fontRendererObj, stack, x, y)
RenderHelper.disableStandardItemLighting();