Soak the gelatin in cold water until soft, about 10 minutes. Bring the currant juice and lemon juice to a boil and remove from heat.
Squeeze the excess water from the gelatin and melt in the warm juice. Line a baking pan with plastic wrap and transfer the jelly into the dish. Chill for at least 4 hours.
Turn the jelly out of the dish and remove the plastic wrap. Cut into diamond shapes and toss in the sugar.