A common puzzle in economics is whether natural resources are a 'curse' or a 'blessing' for economic development. Previous studies have suggested that resource booms can promote growth, but private rent-seeking can turn these booms into a curse if institutions are weak. We argue that private incentives differ depending on whether rents are diversified across different commodities or concentrated in a few of them, because greater diversification implies higher appropriation costs. By using SITC-4 level of export disaggregation to measure within-sector concentration in 131 countries during 1991-2015, we show that the effect of mining rents on economic growth is conditional on the level of concentration within the mining sector. Mining rents enhance growth for economies with low concentration and strong institutions but reduce growth for economies with high-concentration and extremely weak institutions.