It’s a very unique seed where you spawn in a cave in front of some lava. (Note: some have reported that in front of them in the cave they have a waterfall, if that happens for you, create a new world with the same seed.)
Create a walking path with some cobblestone in the lava as seen in the image below.
Soon ahead you will see two emeralds further away.
Continue making a cobblestone path on your right.
When you’ve crossed the lava you’ll see a wall of dirt on your left side. In the bottom-left corner of the wall there is a stone block. If you see that then you know you are at the right place.
That’s where you want to dig. So dig straight forward through the stone and dirt blocks. Don’t dig down yet.
When you see some lava in front of you place down some cobblestone blocks to make a walking path over the lava.
Keep placing the cobblestones until you’ve past the lava and then continue to break the blocks ahead. Soon you should break through the wall and enter a tunnel.
To your left you’ll find 8 diamonds, some redstone and iron.
Seed: glowifics diamonds