Diamonds are not loading correctly

I hope I am doing this right as I have never had a problem before. It seems the diamond texture for the ores isn't loading correctly. I have found what appears to be diamond ore but the minute you go to hit it or mine it the diamond texture just falls off and it is a plain cobblestone block. I feel I can't progress in the game if I can't find diamonds to get the pick axe to mine obsidian to get to the nether. Also when I press F3 to get coordinates or for block level id it's like a -300 which I also don't think is right. I have the ores in the deep slate as gold and silver seem to mine okay but diamonds at either level do not seem to be rendering as the whole block and just the face or two sides of it and then simply flake off when attempting to mine. Any help would greatly be appreciated.