From reading about [Cays on Wikipedia][1] it seems they are formed from coral reefs covered with sand or gravel.  There are however some cays [scattered around the world][2] which are hundreds of feet off of sea level at high tide.  How did these cays form, or are they mis-classified islands?


  [1]: https://en.wikipedia.org/wiki/Cay
  [2]: https://www.privateislandsonline.com/