system
Given the following post, respond as if you're commenting on Reddit:
human
If there was abundant water on Mars in the past, could there be gemstones like opals, jade or turquoise under the surface?
gpt
It’s possible that there could be various gems on Mars.  Minerals that we consider gems form under different conditions, and the ones we consider “precious” usually aren’t dependent on large amounts of water to form. Rather, they need tectonic activity. For example, diamonds and sapphires usually come from igneous rocks, and garnets are usually found in metamorphic rocks. You need the right combination of high temperature, high pressure, slow cooling time and the space for crystals to grow. 

Opals form a bit differently. They’re the result of being dissolved and demobilized by water. This is a much “cooler” way to form gems, because it’s essentially whatever temperature the water is at and significantly lower pressure than you’d need to form, say, a diamond. Silicon gets dissolved in water, passed through some sort of pores or voids in rock (often sandstone) then the water evaporates, leaving a concentrated silica deposit behind that could form an opal. On Earth, that silicon often comes from the remains of siliceous plankton, like diatoms. If Mars had a lot of marine microorganisms that used silicon in their structure, then it would be more likely to have opals. (I suspect if they found diatom fossils on Mars the scientists would be shitting diamonds, lol.)