Mung bean starch
A fine white starch made from mung beans that, cooked with water, sets into a clear, firm and slightly springy jelly.
Mung bean starch is extracted from soaked, ground mung beans and sold as a white powder or in small lumps. It tastes neutral; what matters is how it sets: cooked with water and cooled, it turns glassy, sliceable and elastic without being sticky, which is why it is used for glass noodles and for cold starch jellies cut into strips. Lumps should be crushed finely before measuring so they mix without clumping. It is found in Asian grocery shops, sometimes labelled "green bean starch", or by mail order. It is worth checking the ingredient list, since some packets are cut with potato or tapioca starch and then set softer.
Substitute:
Pea starch comes closest and gives a similarly firm, clear jelly. Potato starch sets softer and more rubbery, tapioca starch turns chewy and sticky, and cornstarch stays cloudy and too soft to slice; none of them is a good substitute for cold starch noodles or jellies, though any will do for simply thickening a sauce.