In my speculative evolution project, I've created a biome where the snowy tops of mountains have been replaced with crystals.
The crystal should come from inside the planet, just like lava.
For the regulars here, yes, my answer to every question lately is a sulfate salt, no, it's not deliberate.
You can get enormous, obviously crystalline calcium sulfate monocrystals, e.g. in The Cave of the Crystals.
To get mountain sized crystals*, you could have a planet that had an ocean saturated with calcium sulfate (or a more soluble or colourful salt. But not a mixture of many types).
*Or at least, mountains made of big monocrystals.
Have the water disappear gradually (I don't propose how, I leave that to you), leaving a supersaturated ocean; like growing crystals in a jar, as the water evaporates, crystal growth occurs. Note: Yes, calcium sulfate is only sparingly soluble. You'd need a water cycle to keep dissolving it from a dry stock or insanely supersaturated waters from the deeps, or something of that sort. Potassium or magnesium sulfate are much more soluble.
A single huge mono-crystalline mountain or mountain range is highly unlikely due to energetic considerations: growing a single crystal requires a very controlled process which is unlikely to happen in a spontaneous way on a scale larger than 1 meter.
On the other hand we have examples of rocks which are actually a cluster of smaller crystals, the most egregious example being granite, which comes from slowly cooled down magmatic intrusions
Granite (/ˈɡrænət/) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.
Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals. Thin section of granite
Granite is nearly always massive (lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history.