4 Pack Natural Gray Slate Coasters. Coasters are constructed of natural slate, which is a fine-grained rock that is easily split into thin, flat sheets. Slate is durable and heat tolerant. These coasters are designed to protect surfaces such as tables or countertops from the heat, moisture, and scratches caused by cups, glasses, or other beverage containers. Slate coasters offer a combination of functionality and aesthetic appeal, making them a popular choice for both everyday use and special occasions. Coaster measures 4” square.