Re: Raised Bed vegetable garden
You are right to want to stay clear of chemically treated materials, and while composites are pretty stable and would last indefinitely, they are not without their leaching components as well.
For all natural, as libcarp suggests, redwood, and other high tannin woods are naturally rot resistant and have a decent longevity in direct contact with the earth. I've got two made from 2x12 douglas fir that are holding up fine (it's what I had on hand ).
If you want something more permanent, you may want to look into pouring cement or using cinder block or retaining wall block. Cement products will last indefinitely and in the case of the latter two, can be enlarged, reconfigured, or removed with very little effort.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!