I created a block model using a glass texture , in the inventory or the players hand, it looks good, but when placed in the world , the glass in not transparent.

Post your code, blockstates and model files.

There are three methods in the block class you need to override:

Render type, is full cube, and is opaque cube

public class YobBlockLantern extends YobBlockBB{

	protected static final AxisAlignedBB BB_NORTH_AABB = new AxisAlignedBB(0.0D, 0.0D, 0.0D,	1.0D, 1.0D, 0.0001D);
	
	public YobBlockLantern() {
		super("wall_lantern",Material.ROCK,"wallLantern",Main.CREATIVE_TAB, BB_NORTH_AABB);
		// TODO Auto-generated constructor stub
	}
	
	@Override
	@SideOnly(Side.CLIENT)
	// choices are : CUTOUT, CUTOUT_MIPPED, SOLID, TRANSLUCENT
	public BlockRenderLayer getBlockLayer()
	{
		return BlockRenderLayer.TRANSLUCENT;
	}
	
	@Override
    public EnumBlockRenderType getRenderType(IBlockState state)
    {
		
		/* choices are:     INVISIBLE, LIQUID, ENTITYBLOCK_ANIMATED, MODEL; */	
        return EnumBlockRenderType.MODEL;
    }
	
    @Override
	 public boolean isFullCube(IBlockState state){
   	 return false;
	 }
	
	
    @Override
    public boolean isOpaqueCube(IBlockState state){
    	return false;
	}
	
}

tried different things for all off them.. Seems odd that it renders correctly in the players hand.

Blockstate

{ 
	"forge_marker": 1,
	"defaults": {
		"model": "yabm:lantern",
		"uvlock": true 
	},
	"variants":	{
		 "normal": [{ }],
		 "inventory": [{ }]
	}
}

 

model

{
    "__comment": "Model generated using MrCrayfish's Model Creator (http://mrcrayfish.com/modelcreator/)",
    "textures": {
        "2": "blocks/glass"
    },
    "elements": [
        {
            "name": "s glass",
            "from": [ 10.0, 6.5, 10.2 ], 
            "to": [ 13, 12, 10.3 ], 
            "faces": {
                "north": { "texture": "#2", "uv": [ 0.0, 0.0, 16.0, 16.0 ] },
                "east": { "texture": "#2", "uv": [ 0.0, 0.0, 0.09, 3.0 ] },
                "south": { "texture": "#2", "uv": [ 0.0, 0.0, 16.0, 16.0 ] },
                "west": { "texture": "#2", "uv": [ 0.0, 0.0, 0.01, 3.0 ] },
                "up": { "texture": "#2", "uv": [ 0.0, 0.0, 3.51, 0.1 ] },
                "down": { "texture": "#2", "uv": [ 0.0, 0.0, 3.5, 0.1 ] }
            }
        }
    ]
}