Mercury(II) iodide - HgI2
Both vials in the picture above contain mercury iodide. Mercury iodide has two allotropic forms and they can easily be transformed into each other. At room temperature, the bright orange/red form is the stable one. Above appr. 125 °C, the orange form changes into the yellow form. On cooling down, the solid does not revert to the orange form and a metastable situation is obtained. With strong mechanical agitation, the yellow form can easily be transformed into the orange form again. Simply rubbing the yellow solid with a glass bar and pressing the powder between the glass bar and the glass wall of the vial makes it change into the orange form. Hefty shaking also slowly transforms it to the orange form.
Mercury iodide is a really beautiful compound. Both allotropes have a very bright color, which is hard to capture with a digital camera. When making pictures of mercury iodide, one has to take good care of lighting conditions, otherwise the color certainly will not be represented well.