I built one a month ago. I had a gas line stubbed up through the concrete decking of my pool when they poured it. 1.I got a 90 degree shut off valv and connected it to a flexible gas line (pick one up in the appliance section at local Home builder store). 2.Bought a fire ring from an outdoor furniture\BBQ store and connected the fir ring to the flexible gas line.(but you can buy the kit here. http://www.firelogs.com/HearthProductsFireRing.htm) scroll down the page. 3. I bought stackable blocks (like you would use to build a small garden retaining wall) and configured them into a circle (abot 15 blocks per layer, 3 layers high). no need to morter, just stack 'em. 4. Bought 10 bags of lava rock in the BBQ section of home builder store and poured them in under and then over the fire ring. Now I just turn on the gas valve and light it. Works great. If you want pictures, let me know by email. rgs@thesissons.com

My answer is based on common sense knowledge, not scientific laboratory or engineer's testing. If someone comes up with an answer from either of those departments, I'll be happy to defer. I think of how many parts of a fireplace surround or hearth of various natural stones or concrete. As long as you're not in the direct "firebox" or chimney area, I don't think the material will be exposed to sufficient heat to cause harm. Keep in mind that the O2 being supplied to the fire is coming from the outside air -- cool air -- washing over the fire pit wall and cap on its way to the fire, ameliorating the effect of heat somewhat. That said, my educated guess could be wrong because that's how educated guesses sometimes are. Hopefully, my answer will not discourage anyone from offering a better one, if they have it.

We want fireproof balls for this existing fire pit

If you're using segmental wall block, you're looking at digging down 6" for the gravel footer + at least half a block below grade. At that point you may end up getting close to conflicting with the lines, depending how shallow they are on your property. And when doing a built-in firepit we recommend trenching a drainline out to daylight so that rainwater doesn't pool inside it, which - I don't trench across drainfields. I agree w/ Yard re: talking to septic pros, but... I'd also consider other options. Personal bias - I don't like the look of lego block firepits. If you want something with some oomph, google Firekettles. We did one for a client and they love it. 4 ft diameter firepit with a rail all the way around for a footrest, it's awesome. In your case, the fact that it's freestanding and portable - if you can call something that took 4 guys to move "portable" - is a bonus in terms of 1) not conflicting with the septic lines and 2) being movable if the drainfield needs dug up for any reason. As long as we don't find any surprises when we renovate the bathroom next month, I'm looking to buy a firekettle in the spring :)