12 gray rubber bottle stoppers. These stoppers can be used to make self-healing injection ports in Mason jar lids or medicine bottles. Mainly used for mushroom growing and liquid culture production. Use with hypodermic needles to inject cultures, therefore never needing to remove the lids on your culture jars. They fit 13 mm (0.5118’’) diameter opening. They are 3/4” across the top, and 5/16” top to bottom. Simply use a 7/16 to 1/2 inch drill bit and drill a hole in metal or plastic jar lid and the stoppers pop right in place.