The Amethyst geodes in Minecraft are covered in unique blocks you can only find there. Smooth Basalt is one of those blocks that naturally spawn around it. The thick, outer layer surrounding the Calcite, the white layer, and the darker purple Amethyst, which features budding blocks that form Amethyst clusters with Amethyst shards, and regular Amethyst.
Beyond finding Smooth Basalt surrounding the Amethyst geodes, you can also make it. All you need to do is find regular Basalt blocks, bring those to a furnace, have some fuel prepared, and smelt it down. Your Basalt block will transform into Smooth Basalt, and it should have a slightly changed appearance.
Of the two options, finding Basalt will properly be easier than locating Smooth Basalt in the wild. You can only find it surrounding Amethyst geodes, and those are pretty rare. An Amethyst geode typically spawns between Y=70 and bedrock. Regular Basalt blocks regularly spawn in the Nether. If you have access to the Nether, you have a better chance of locating it there pretty frequently, but if not, you might want to try your chances of locating an Amethyst geode in your travels.
Once you have your Smooth Basalt, it’s primarily used for decoration. It is not required for any crafting recipes.