Most often used in advanced crafting, Crystal is an uncommon to rare resource found in Ark. It is most often found in dangerous areas, such as moutain tops, the plains in the snow biome, and in caves.
Crystal is easiest to find at night or in low light. It is self-illuminating, meaning that it glows from within. The bright light produced within crystal makes it stand out exceptionally well against darker settings.
The most common places to find Crystal are on many of the mountain tops, scattered across the plains in the snow biome, and throughout many of the caves on the Island. While this may sound quite dangerous, there are a few cases where you can get your hands on some crystal without much risk.
If you are on the East Coast of the Island you may well be able to venture into the edge of the snow biome near the shore to harvest some crystal. Be careful though, as the extreme cold will cause you steady injury if you aren't wearing Fur Armor. There are also a number of dangerous critters running around in the snowy regions.
Perhaps the easiest way to get your hands on some Crystal with very low risk would be to poke your head into a few of the caves on the Island. I've found crystal deposits just inside some of the caves before encountering any hostile life or dangerous enviromental conditions. I walked in, harvested and left. Wait for the resources to respawn and repeat. Some of these caves can also be found in very low threat areas. Just don't push your luck and venture too deep - unless, of course, you are ready for cave diving and adventure!
To effectively harvest Crystal you will need a Metal Pick or Ankylosaurus (Ankylo). In the caves you're pretty much restricted to a Metal Pick, but in the snow biome or on the mountain tops you can have at it with an Ankylosaurus. If you find that you don't get much Crystal, or that many Crystal Nodes aren't dropping anything but stone, that is normal. Just keep going with either of the above mentioned harvesting methods and you'll end up with Crystal. The drops are small and are not guaranteed from every node each time.
Once you have found a Crystal location that is handy for you, make sure you mark down the location. Crystal respawns in the same place, so once you've found your sweet spot you can simply return there on a regular basis to keep harvesting.