![French Egyptian Revival Chrysoprase and Plique-a-Jour Enamel Necklace. French Egyptian Revival Chrysoprase and Plique-a-Jour Enamel Necklace.](https://www.langantiques.com/university/wp-content/uploads/elementor/thumbs/french-art-nouveau-silver-chrysoprase-plique-a-jour-pendant_2_90-1-11309-q88jyye3w98vx6kxtpnh4scwy4f58mz2j8jj9v36vc.jpg)
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 for microcrystalline and cryptocrystalline materials. The latter two definitions are absolute due to the high magnification powers of modern microscopes. The included illustration is an example of a polycrystalline gemstone: chrysoprase.