Agates occur in nearly every state, along with countries around the world. Their patterns are endless and often striking, sometimes unbelievable. Right now there is a great agate thread going on on the open Facebook group Rockhound Connection:
Make sure to check out the posts. They are all variations of quartz.
As beautiful as some of these cut and polished specimens are, many beginners are confused as to what to look for. Although not always present, a certain translucence and a wavy character to the rock are good signs. Some agates are so outrageously striped that there is no doubt as to what they are.
Here is a video of an agate that I liked so much that I have never had the heart to cut it open. The second photograph shows another agate from the same location, one I cut into a slab with a rock saw.
Both of these rocks are for examples only, they did not come from the Southwest. But if you are ever in Northern California, you may want to check the riverbed of Cache Creek in Yolo County. Good luck.
Uncut agate. Click on image or text link below. (Movie)