The 55 mm Stackable Filter Adapter from SRP is made from CNC machined Delrin®.  The adapter allows you to stack the 55 mm Marumi Macro +10 lens onto either a color correcting filter or another macro filter.  Stacking multiple macro filters is used to decrease the focal distance of the GoPro® camera.  Although macro filters do provide some magnification when used with a GoPro camera, the main benefit is that the addition of macro filters will allow you to get the camera closer to the subject.  The screen shots below of a US penny were all taken by a Hero3+ in 1080p Narrow mode in the configurations listed under the image: