package ljfa.glassshards.render;

import org.lwjgl.opengl.GL11;

import net.minecraft.client.renderer.ItemRenderer;
import net.minecraft.client.renderer.RenderBlocks;
import net.minecraft.client.renderer.Tessellator;
import net.minecraft.entity.item.EntityItem;
import net.minecraft.item.ItemStack;
import net.minecraft.util.IIcon;
import net.minecraftforge.client.IItemRenderer;

public class TransparentItemRenderer implements IItemRenderer {

    @Override
    public boolean handleRenderType(ItemStack item, ItemRenderType type) {
        return type == ItemRenderType.ENTITY;
    }

    @Override
    public boolean shouldUseRenderHelper(ItemRenderType type, ItemStack item, ItemRendererHelper helper) {
        return helper == ItemRendererHelper.ENTITY_BOBBING
                || helper == ItemRendererHelper.ENTITY_ROTATION;
    }

    @Override
    public void renderItem(ItemRenderType type, ItemStack item, Object... data) {
        if(type == ItemRenderType.ENTITY) {
            //RenderBlocks render = (RenderBlocks)data[0];
            //EntityItem entity = (EntityItem)data[1];
            
            IIcon icon = item.getIconIndex();
            
            float f = icon.getMinU();
            float f1 = icon.getMaxU();
            float f2 = icon.getMinV();
            float f3 = icon.getMaxV();
            
            GL11.glEnable(GL11.GL_BLEND);
            GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
            ItemRenderer.renderItemIn2D(Tessellator.instance, f1, f2, f, f3, icon.getIconWidth(), icon.getIconHeight(), 0.0625f);
        }
    }
}

package ljfa.glassshards.render;

import net.minecraft.client.Minecraft;
import net.minecraft.client.renderer.ItemRenderer;
import net.minecraft.client.renderer.Tessellator;
import net.minecraft.client.renderer.entity.RenderItem;
import net.minecraft.item.ItemStack;
import net.minecraft.util.IIcon;
import net.minecraftforge.client.IItemRenderer;

import org.lwjgl.opengl.GL11;

public class TransparentItemRenderer implements IItemRenderer {
    
    @Override
    public boolean handleRenderType(ItemStack item, ItemRenderType type) {
        return Minecraft.getMinecraft().gameSettings.fancyGraphics &&
               (type == ItemRenderType.ENTITY
            || type == ItemRenderType.EQUIPPED
            || type == ItemRenderType.EQUIPPED_FIRST_PERSON);
    }

    @Override
    public boolean shouldUseRenderHelper(ItemRenderType type, ItemStack item, ItemRendererHelper helper) {
        return helper == ItemRendererHelper.ENTITY_BOBBING
            || helper == ItemRendererHelper.ENTITY_ROTATION
            || type == ItemRenderType.ENTITY && !RenderItem.renderInFrame && helper == ItemRendererHelper.BLOCK_3D;
    }

    @Override
    public void renderItem(ItemRenderType type, ItemStack item, Object... data) {
        IIcon icon = item.getIconIndex();
        
        float minU = icon.getMinU();
        float maxU = icon.getMaxU();
        float minV = icon.getMinV();
        float maxV = icon.getMaxV();
        
        if(type == ItemRenderType.ENTITY) {
            GL11.glTranslatef(-0.5f, -0.25f, 0.0421875f);
        }
        
        GL11.glAlphaFunc(GL11.GL_GREATER, 0.1F);
        GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
        GL11.glEnable(GL11.GL_BLEND);
        ItemRenderer.renderItemIn2D(Tessellator.instance, maxU, minV, minU, maxV, icon.getIconWidth(), icon.getIconHeight(), 0.0625f);
    }
}

Add to the renderer