Can't mine diamond in Minecraft?

I've tried using both iron and diamond pickaxes, but diamonds just go poof into smoke. I'm playing survival, and no - the diamond is not falling into lava. Is there any way to fix this? I'm using version 1.12.2, and the only mods I have are a texture pack (Conquest), Optifine, LiteLoader, and VoxelMap. All these work fine.

Quoted from Faederwolf in the comments:

Hmm, seems there were no diamonds and the texture pack mistook gravel for diamonds. Problem solved.

A good way to avoid this is one piece of info given to you by viewing the screen info that includes co-ords, usually accessed via pressing F3. It also tells you what block you are looking at.