"Rainbow Rock"- the detail with the perspective on the side facing away is a nice touch. If only the same care had been put into making the gradient more organic. plus a small, selective color palate is much more attractive than an easy rainbow.
Mineral crystals grow under a variety of conditions. Slow cooling of magma (melted rock) deep within the Earth generally leads to a rock such as granite with visible crystals of feldspar, quartz, mica, amphibole and other minerals.
Topaz (Aluminum silicate fluoride hydroxide) is a common gemstone that has been used for centuries in jewelry. Its golden brown to yellow color is classic but is confused with the less valuable citrine, which is sometimes sold under the name topaz.
Ethiopian opal is still a new gemstone. Ethiopian opal is quite unique and different from more common Australian and Mexican opal. Not many people know about this opal yet. I am a gem cutter and have worked.