Sorry I'm late to this one. First to correct the obvious misinformation.
PEX can't be used for natural gas in any way shape or form. And just to get this out there, plastic pipe of any kind is prohibited if used above ground. Water line like PEX or PVC is prohibited period and (as someone who works for a gas company) if it is noted downstream of your gas meter by your gas company they will probably be forced to shut off your gas until you replace it.
Buried black iron was the state of the art way to do this job until about 1980....but we've moved on with materials since then. Not that it won't work....but it will eventually rust, develop leaks, fill with water, and quit working. How long that will take depends on soil conditions. I've seen it last 30 years. I've seen it fail faster. It's still allowed by code. Copper may be allowed. You'd have to verify with your gas company. National fuel code prohibits its use if the sulfides in the gas stream are above a certain threshold. I've seen a lot of issues with it clogging due to internal corrosion even when sulfides are below the NFC spec though...and as it is expensive as well....I wouldn't use it even if allowed by your gas company.
The correct way to run underground gas now is with approved high density polyethylene gas line. You can buy it online and at plumbing supply stores. Joints can be made DIY without special tools if you get the right stab connectors. For each end you will need anodeless service risers. These are assemblies that you will connect horizontally to your underground piping, they sweep a long 90 upward and they terminate as NPT threaded steel. Use a valve at the start of all this so you have a good place to shut it off and isolate it going forward. Bury a 12 or 14 gauge coated wire with it so anyone with a pipe/cable locator can find it for you in the future. The tracer wire should come above ground at the risers and just wrap it a few times around the riser or zip tie it so you can find the end if you need to.
That's essentially the same way natural gas service lines are done to houses these days (at least 95% of the time). The PE pipe is rated at 80 to 100 PSI in gas service...but you will likely only be running it at about 0.25 PSI in your application. 3/4" pipe would work for your distance and load.
Good luck.
Phil
p.s. Here's an example of the type of material I would use if it was me.
https://www.homedepot.com/p/HOME-FL...rs-Gas-Line-Detection-18-429440-007/309182788
$184 for 100 feet and everything you need for the underground part. I'd bury it at least 18 inches deep.
