I’ve never made a spell jar myself, but I’ve seen quite a bit of discussion about safe, eco-friendly practices for burying spell jars in the ground and I have a tip.
Stone- or earthenware clay is what potters use in making ceramics. One of the nice things about clay is that it’s still biodegradable until you fire it, which means that it can be broken down and reused if you decide you don’t like your piece. You can recycle scraps left over from projects and you haven’t lost anything.
So a witch who’s looking to bury a spell jar without leaving glass or other invasive materials in the ground could make a pinch pot out of clay, fill it with whatever ingredients are desired for the spell, and bury it without fear or shame. Assuming the ingredients in the spell are biodegradable, then everything, including the clay jar, will simply degrade over time and return to the place whence it came. And besides, what better way to connect instantly with earth energies than using a jar made of something dug directly from the earth?
If you have a local pottery studio near you, you can sometimes purchase bags of reclaimed clay for MUCH cheaper than a new bag - at the studio where I make my ceramics, a bag of reclaim is $9 as opposed to $20.
(Also, please note that this only works with art clay. Do not use a polymer clay like Sculpey or Fimo!)