Polycrystalline substances are minerals whose outer shape is not recognized as a clear crystalline form. The crystalline form can only be recognized under high magnification. It is a grouping name of microcrystalline and cryptocrystalline materials. The latter two definitions being absolute due to the high magnification powers of modern microscopes. Below you can see a good example of a polycrystalline gemstone: chrysoprase.
French Egyptian Revival Chrysoprase and Plique-a-Jour Enamel Necklace.