According to the image of Sunstone Island on the Dragons: Race to the Edge Interactive Map, it has a lot of vegetation and perhaps large old trees, based on the single tree stylized on the map image. It is said to have beautiful shores and a variety of mineral deposits.
In "Appetite for Destruction", the Dragon Riders survey dragon populations on several islands. The Riders have marked the dragons of Sunstone Island with orange paint. As the Riders each mark dragons, they realize that dragons from one island are ending up on the others. They headed for the island, only to realize that it had been reduced to a small lump of rocks with a single pine tree, barely enough to fit three dragons. They discovered that these islands, including Crescent Island, Thor Rock Island and Tall Tree Island, are being systematically destroyed and the dragon populations are migrating. A Screaming Death is responsible, destroying islands as it makes its way to Berk.
Historically, there is an as-yet unproven theory that Vikings used sunstones to help navigate their longships. Sunstone is a mineral that is able to polarize light, so in theory a Viking at sea could look through the stone and find out where the Sun was located on a cloudy day.