In a small saucepan, bring the sugar and 1/2 cup of water to a simmer over moderate heat, stirring until the sugar is dissolved. Set aside until slightly cooled but still very warm to the touch. Stir in the rose petals, packing them into the syrup as they wilt. Using a rubber spatula, scrape the mixture into a glass measure or small heatproof glass bowl. Cover with plastic wrap and set aside to steep at room temperature for at least 6 hours, or refrigerate overnight.