On a recent trip to Pingtung, we dropped into the Shell Beach Exhibition Hall (砂島貝殼砂展示館) and I learned the following:
The sandy beaches and mudflats on the west coast were created by shale washed down rivers from the Central Mountain Range; many are quite grey. However, most of the beaches in the south (including those on Little Liuqiu) are crushed coral or shell beaches. North Taiwan's beaches are generally quartz sand, while the east coast is rocky, the boulders being metamorphic material washed down from the Coastal Range.
Shadao Beach, pictured above, is next to the hall. It's a nature preserve and the public can look, but not set foot on it.