Painting gemstone and crystal effects requires understanding something that took me a long time to grasp: gemstones are primarily characterized by what they transmit, not by what they reflect.

Most objects we paint are opaque. Light hits the surface and either reflects or is absorbed. The color we see is the surface color.

Gemstones are translucent. Light enters them, reflects off internal facets, and exits in different directions. The colors we see include both surface reflection (which is largely the ambient light color, like metal) and transmitted light (which carries the stone's color from within).

The practical painting approach: the dominant effect in a cut gemstone is the light that has entered and been redirected by internal facets. High-saturation color in the areas where concentrated light exits, dark in the occlusion zones, and a sharp surface specular where the ambient light reflects.

The color temperature variation inside a stone is often the most beautiful part. A sapphire's internal lights shift between cool blue and near-white depending on the light path through each facet.