Since Rosentia Island isn’t far from the spine of mountain ranges, when I began looking at mineral resources I started researching volcanic rock. I settled on granite as I used to live near a Granite Island and knew a bit about the texture and look of granite. Since the mineral resources of Rosentia Island is simply background material, I used the internet to dig up some basic information on the formation of granite and some of its shapes (i.e. tors and rounded massifs). Books are better for more in-depth and precise information. I also researched what other rock might develop alongside granite but have so far had no luck.
The real world Granite Island, despite being both smaller and uninhabited, became a starting point for my Rosentia Island. I researched its geological beginnings to understand how Rosentia Island might have been shaped. The size of the island was determined through considering how long I wanted a journey across the island to take (2 – 3 hours from west to east).
The story needed both stretches of wilderness and a settlement that had lasted centuries. Some dark secrets have kept down the population but it needed something more to justify the expanse of wilderness. Thus I made it a harsher island. The rockiness in the middle defeats most attempts at crop lands and helps protect a few stretches of the open woodlands to the far west. The western and central parts are also higher above sea level and more hilly even in the wooded parts. This is also keeps a large stretch of the island relatively unpopulated as most can’t be bothered walking far up-hill. To reduce the impacts of deforestation I not only gave them a re-planting cultural custom (if you’re not clearing for farmland, replant for tomorrow) but also decided that in a bushfire-prone country, they would build their houses out of stone. It also helps that the temperate climate keeps that many seasons warm enough that fire places are unnecessary for much of the year.
Wikipedia can also be handy for designing islands and mainlands by looking at some of their categories. Check out coastal geography: http://en.wikipedia.org/wiki/Coastal_geography