4. Opal
Since we have noted that silicates are most common minerals, some of the easiest gemstones to find in rocks are exactly silicate gemstones. One of them is opal, amorphous silicate mineral. It occurs in many rocks as a product of low-temperature hydrothermal processes. There are many opal varieties, and the main opal gemstone resources are found in Australia.
Pixabay/Public Domain
