Minecraft wiki entry for Ancient_Debris, associated text: The texture of the ancient debris is supposed to look like pressed metal plates.[1]
Ancient debris is actually what remains of the mining activity of the piglins, who extracted all the original netherite ore.[2]
It is possible to find blobs of up to 20 contiguous ancient debris - up to five debris can generate in a chunk, so if a blob of five generates in adjacent corners of four different chunks, a blob of 20 debris would be created.
Despite having the same blast resistance as obsidian, ancient debris can be moved by pistons.