This post ear nut component is made of plastic, and is 4mm deep and 4mm wide. It has a 1mm hole for the post. Made of a rubbery soft plastic so they are very comfortable. Most posts are 20 to 21 gauge and will fit perfectly into this ear nut. They work well with the stainless posts in my shop.
This makes a great earring back for all of your post earrings.