Elastomer is a durable, hypo-allergenic soft plastic free from phthalates. Use water-based lube and wash them with soap and water or toy cleaner. Disposable after a few uses.
They soften the vibration a little bit, and add some texture. These are made out of Elastomer which is porous and will degrade over time, but you can get 2-3 uses out of it if you wash it with warm soap and water. These are pretty similar to Tenga Eggs. LeWand also now makes silicone texture covers, which are non-porous and extremely durable!