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Igneous rocks formed on the surface cool down in just a matter of a few hours.
The crystals in these rocks can be microscopically small.
Lapis Lazuli Gemstones: As a general rule, solid blue lapis or solid blue with a few grains of gold pyrite are the most desirable colors.
In the photo above the bottom two cabochons approach that ideal.
Silicates are minerals that contain silicon and oxygen.
Over 90% of the Earth's crust is made up of silicates.
The rest of the minerals are lumped into a group called non-silicates.
The top left cabochon has large patches of calcite that are intergrown with blue lazurite to yield a faded denim color. For most people, it would be the least desirable stone in the photo; however, some people will enjoy it.
Desirability in lapis varies from stone to stone and from person to person.