Ok, I'm not scared to go against the grain here.
OP, I'm from your area.
Never the same 'man with lance' motif like yours, but I've seen these pendants at the town square market days, the flea markets, and (unfortunately) I've even seen them for sale in the past at a respectable lapidary shop in Wimberley.
I have also seen them made... and it's always seemingly the same 2 types of guys. The everyday "keep austin weird" tie-dyed shirt wearing liberal arts major hippy, or the 60+ year old dude who enjoys sitting in his shop chair and working with his hands enjoying his retirement...
One of these types seems to sell them as a novelty, the other on occasion tries to pass it off as a GEN-U-WINE artifact to the out-of-state tuiton types who can be found hanging out at Barton Springs or debating IPA brews down on Congress/Red River, etc.... I'll let you decide which one is which.
They usually use semi-professional tooling which is impressive in its own right... on one occasion i witnessed it done the guy was using actual pre-Ingersoll Rand swedish made ARO tooling with a top of the line air drying system. Turns out the guy was retired out of the fossil preparation field, and he was truly an artist.
He started with an air scribe, hit after it with dental burrs and jewelers abrasive drums. Then he moved on to microblasting amd eliminated his tooling marks, and whenever he got it how he liked it, it was thrown into the media tumbler where it was left until he was happy with the wear/fade.
After he had 1-2 dozen he would take them and sell them to the lapidary shop... To finish it off, they threaded it onto a leather choker-type cord... I'm not sure but I think I remembered seeing them on key rings or zipper pulls too.
I've seen a limited few examples of these type pendants over the last few years actually around people's necks too - The 'survivalist' types seem to like them as well... i guess for when they are out and about with their tactical backpacks, paracord bracelets, and extremely stupid Cold Steel knives. Perhaps that is why you found yours in a tire rut... that cheap Chinese "leather" cord finally gave way on a hike or whatnot.
I guess it's a cool find if you like it for what it is, or take into account that it is handcrafted and possibly one-of-a-kind...?
Anyways, thanks for hearing me out.
Happy Hunting!
~Tejaas~