Gemstone painting for dummies Also applicable for plastic

Coloring

Add base color with Ctrl+U (In Photoshop)

The method mentioned in this tutorial could be used on more than gemstones. Feel free to try this method out!

Use Color Range to select the light gray areas, duplicate the layer, and adjust it using a multi-peak curve to create some subtle colors (similar to creating a laser material effect).

Adjust the base layer's color using Curves as well.

Change the blending mode of the selected light gray layer to "Multiply" and merge it with the base layer.

After merging the layers, use the Dodge Tool to enhance some small highlights (basically complete at this point).

Use the Smudge Tool. Soften some edges, then add some small star-like sparkles, and you're done.

The same method applies to coloring natural crystalline gemstones.