Jeffrey Anderson is describing this location on his website dwarve´s earth treasures:

“...The agate fields around Balmorhea Lake is well known to the collectors for their strong blue colored agates as well as black plume agates and the classic example would be one with strong white and blue color combos. Lake Balmorhea is mainly a fishing site and also a great birding site where many migrating birds stop by to rest. The flat areas to to the south of the lake will produce bleached agate fragments and it's possible to find larger fragments and the washes are best places to look for the agates. Some agates are also found in the hills on the northern side of the lake. I didn't had a much of luck on the southern side of the lake. I followed small agate nodules on the northern side and was lucky to have hit on an agate bearing basalt beds where I by chance chiseled what I thought to be a small agate only to discover that I have exposed a blue agate much larger than I expected. Ever since that first in-situ agate I have found, I continued to find more agates including those with plume inclusions. I also found a calcite specimen with plume inclusions, a clue that plume minerals were formed independent of any agate genesis.
Since the area around the lake is a private land that is open to recreation including rock-collecting, anyone wishing to try their luck at finding any agates must stop by the fishing store to pay $4.00 fee before venturing out. Just take a route toward the Lake Balmorhea and keep going till you see the white fishing store on the right side.
I do wonder if the agates of Balmorhea share the same basaltic volcano systems as those of Marfa agates since they show similar colors.
I learned that there are more agates found around Lake Balmorhea and most are weathered and bleached but can show banded agates and plumes...”