If you know diameter or circumference, estimate volume and use water as the "composition" to get a first estimate of weight. Composition will be "heavier" than water so you might increase the estimate by 10 to 20%. You could estimate density by putting them in a denser liquid and compairint their bouncy to a known mass. You could dry them and weigh them if you have a sensitive balance and then estimate % dry matter and do a reverts calculation to get weight.