Homemade Fire Pits. From Homesteading/Survivalism Page on FaceBook

How To Make A Fire Pit for Around $50.
Materials needed:
Weber Smokey Joe portable charcoal grill or equivalent 14" diameter grill. ($30 new. http://amzn.to/12HbWt0
)
4 sections of 14" inside diameter concrete tree ring ($2 to $3 each = $8 to $12 total)
6 sections of 24" inside diameter concrete tree ring ($2 to $3 each = $12 to $18 total)
2 cubic feet of small stones, pebbles, road gravel or decorative rock ($0 to $20 depending on how fancy)
Total cost $50 to $80 depending on your taste in stones.
Full instructions: http://tinyurl.com/m2l4hak
　
　
Find a nice level area of your yard or create a level circle approximately 3 feet in diameter. It's not absolutely necessary but we sprayed our pit area with weed and grass killer to make a bare spot. You will notice the ring of browned grass surrounding the pit in the final photos. This is due to the weed killer and not the result of heat from the fire. We also placed a layer of weed barrier cloth under the pit to prevent grass/weed from growing up into the pit. The tree rings will be more stable on bare earth than on grass, particularly if you have Bermuda grass like we do. Also, you should have no problem if you want to place your pit on top of a concrete or brick patio.
The trick to turning tree rings into a decent looking fire pit is to make the ring two sections tall by turning the fluted top sections upside down so they interlock with the fluted bottom sections. The first photo shows what the 14" tree ring sections look like when you buy them from the store and the second photo shows them stacked. They don’t fit perfectly but the small air gaps look sort of decorative in my estimation and are barely noticeable once the unit is being used.
　
It is VERY stable. I think that is due to a combination of the weight of the ring sections, the friction of concrete on concrete, and the "tab and slot" connectors at the ends of the 24" outer ring sections. However, if stability is a concern you can use Loctite Landscape Block Adhesive (under $5 at Home Depot) to glue the sections to each other. This makes the location of the pit rather permanent but with some work it could still be moved from one spot to another.
To be certain it will work, go to your Home Depot and take your tape measure with you. Lay the rings out on the floor and measure the inside diameter. Then go to the bar-b-que grill section of the store and measure the diameter of the charcoal grills. That should tell you if you can find the right fit. I'm guessing the 12" rings you refer to are actually the 14" rings I used for the project. But you can quickly determine this by laying out the blocks and measuring. If the inside diameter is 14" then you are in business. I think the larger 32" blocks for the outside ring would make an even nicer pit that the one in this instructable. All you'll need is more stone to fill the larger void between the inside and outside rings. Let us know what you discover.

The browned grass in the photos is totally the result of my spraying the area prior to putting up the pit. We purposely wanted to kill the grass and potential weeds in that area before setting up the pit. With a fire in the Weber, there is virtually zero temperature rise in the outer ring. You can easily touch it with your hand or sit with your feet up on the outer edge. Also, the Weber is a good 6 inches off the ground with a two inch layer of stone below that. So there is very little heat down at ground level
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Loved your idea and wanted to do something similar. Since I live in Spain, those tree rings are hard to come by and a bit expensive, however, terracotta planters are in abundance and relatively cheap. So, I took and modified your idea and I think it turned out really nice.