Preheat the oven to 225°F. Line a baking sheet with parchment paper.
In a small bowl, stir together the salt, lemon zest, and lime zest. Pour the salt into two slightly mounded piles that run the length of the baking sheet. Bake for 60 minutes, or until the salt and the zest start to brown slightly. Remove from the oven, and let the salt cool on the baking sheet.
You can leave the salt coarse, or transfer the salt to a food processor or spice grinder and pulse until ground fine. Transfer to an airtight container and keep for up to 2 months.
Recipe courtesy of Wonderful Citrus.