Comment by Lanrei

The article says that they're biconcave, which means that the thinnest part of the lens is in the centre (similar to a red blood cell). For high-myopia lenses, they often remove the lens material away from the centre to limit the maximum thickness at the expense of reducing the field of view. The resulting surface is usually left unpolished, but the lens underneath will still bend light, resulting in the circular patterns you mentioned. In this case a very large amount of material is removed as they would be impossible to use with a conventional frame otherwise. The alternative would be to use glasses with very small lenses.