Single Dravite crystal exhibiting the 'Usambara effect' color change (named after nearby mountains). This is a different type of color change than the Alexandrite effect, where the color changes in different types of lighting. In this case, the Dravite crystal appears green in normal lighting, however when a single light source is transmitted through the crystal, at a certain thickness, a sudden color shift from green to red is observed, while less thick areas remain green. Well terminated, complete crystal. This is often demonstrated by taking thin slices of crystals. If, for instance, the critical thickness is 3mm, you can take two slices that are 2mm thick each, and each will appear to be green. If you overlap them slightly, creating a combined thick ness of 4mm the area that is overlapped will appear red. This crystal is dark but very transparent, gem quality.Transparent red with strong backlighting.

Single Dravite crystal exhibiting the 'Usambara effect' color change (named after nearby mountains). This is a different type of color change than the Alexandrite effect, where the color changes in different types of lighting. In this case, the Dravite crystal appears green in normal lighting, however when a single light source is transmitted through the crystal, at a certain thickness, a sudden color shift from green to red is observed, while less thick areas remain green. Well terminated, complete crystal. This is often demonstrated by taking thin slices of crystals. If, for instance, the critical thickness is 3mm, you can take two slices that are 2mm thick each, and each will appear to be green. If you overlap them slightly, creating a combined thick ness of 4mm the area that is overlapped will appear red. This crystal is dark but very transparent, gem quality.Transparent red with strong backlighting.

Deep red, translucent twinned Cinnabar crystals on matrix associated with Quartz crystals. Hard to find specimen from an uncommon old locality.

Single yellow scalenohedral Calcite crystal with a sharp phantom of Cinnabar. This is a rare specimen from the old mercury mining district of Terlingua, located near the Mexican border. Cinnabar was discovered here in the 1880s, creating a temporary mining town, now nothing remains but a ghost town.