Igneous Rocks: These form from the cooling and solidification of molten rock (magma or lava). The rate of cooling significantly affects crystal size:
* Intrusive Igneous Rocks: Cool slowly underground, allowing large crystals to form. Examples: granite, gabbro, diorite.
* Extrusive Igneous Rocks: Cool quickly on the surface, resulting in smaller crystals or even a glassy texture. Examples: basalt, rhyolite, obsidian.
Sedimentary Rocks: While less common than in igneous rocks, some sedimentary rocks can contain crystals. These crystals are often formed from minerals that precipitate out of solutions in water. Examples:
* Evaporites: Rocks like gypsum and halite form as water evaporates and leaves behind dissolved minerals that crystallize.
* Chemical Sedimentary Rocks: Rocks like dolostone and limestone can contain crystals of calcite or dolomite formed by chemical reactions.
Metamorphic Rocks: Metamorphism (heat and pressure) can cause existing crystals to grow larger or change their form.
* Foliated Metamorphic Rocks: These rocks, like schist and gneiss, often have visible crystals due to the alignment of mineral grains during metamorphism.
* Non-Foliated Metamorphic Rocks: Rocks like marble and quartzite can contain large crystals, though they aren't always visible to the naked eye.
Important Note: Not all rocks with crystals are easily identified as such. Some crystals are microscopic, while others can be massive. However, if you're looking for rocks with visible crystals, igneous rocks are a great place to start!
