While you can find obsidian in the Nether by looting chests, you shouldn't rely on this. Without diamonds, you can't stockpile obsidian, only make it as you need it. If you get lost and can't find your way back to the original portal, you're basically out of luck. While you theoretically gain access quicker than other players, you are unable to build another nether portal in the Nether.

Keep in mind, there's one major flaw with this approach. If everything is correct, the nether portal should be filled with an ominous purple glow.

Finally, take your flint and steel and light one of the bottom corners of the nether portal.

If something's not right, you may need to repeat these steps in a different location. Check to make sure your measurements are correct. Clear away all your excess building materials to reveal your nether portal. I recommend either building a closed space for it or keeping it separate from everything else since it is possible for Nether mobs to travel through a portal occasionally.

Find a suitable place for your Nether portal.

To build a nether portal without diamonds, do the following: Just make a mold to make sure the obsidian happens to be created in the exact shape needed for a nether portal. The secret to building a nether portal without diamonds is relatively simple: obsidian is formed when water hits a lava source block.

Once you have that out of the way, follow the steps below, and you'll have a perfect portal in no time. First off, make sure you're familiar with the basic build of a nether portal by referring to the section above. Once you've rounded up all your materials, it's time to get started. Assuming you chose cobblestone as your material of choice, you'll need a pickaxe to mine it when you're done. Source: Windows Central (Image credit: Source: Windows Central)

Obsidian portal