I really love amethysts in minecraft, and I think it a great combination - Nether Portal and Amethyst Geode.
A few vines and moss make this structure very natural, and Crying Obsidian smooths the transition from bright crystals to the subdued radiance of the obsidian frame of the portal.

This portal was built for my survival on one server, and this is a secret entrance to my base, and later a secret piston door was added to close the passage.