Try checking what kind of file it is and double check and make sure in your marking material file it's the correct.
It might be jpg when it should be png.
Double check and make sure it's written [in the Marking material notepad file] as .png
I put together a DiamondGem.png texture. It will look slightly different than others' who have the texture, but it looks very nice imo. Hope this helps!
https://www.mediafire.com/?mqbuej9bdw82qkm (https://www.mediafire.com/?mqbuej9bdw82qkm)
Place in your items folder.
(http://i.imgur.com/jLsiCCl.png)