Beach sand contains plenty of the mineral quartz. Why should you not put your plastic sunglasses down on the sand at the beach?
quartz is hard, has sharp edges, and will scratch plastic.
You should not put your plastic sunglasses down on the sand at the beach because sand, particularly beach sand, contains plenty of the mineral quartz, which is incredibly hard. Quartz has a Mohs hardness scale rating of 7 out of 10, which means it is harder than plastic. When you place your plastic sunglasses on the sand, there is a high chance that the grains of quartz in the sand can scratch or damage the plastic lenses of your sunglasses.
To avoid this potential damage, it is recommended to use a protective case or pouch for your sunglasses when not in use. These cases are typically designed to provide a soft and safe environment for your eyewear, preventing any potential scratches or harm. Additionally, you may consider placing a towel or cloth under your sunglasses when putting them down on the beach to provide an extra layer of protection.