This is either a short story or a "shortish" novelette, I am not sure. I read it about 20 years ago but probably in a collection, so it could be much older.
It happens in a world quite similar to ours, but for one point. There is a country with two ethnic groups. Of course each group has a name but I forgot what the names were, so I'll call them AAA and BBB. I think the title was "BBB ovens" but I got nothing, not remembering what BBB was.
For many centuries, AAAs were city dwellers, rather rich and proud of their intellectual prowesses. BBBs were mostly ignorant peasants, very poor. But the BBB had an advantage: they did not need fuel. They knew about two kind of stones. With one kind they built ovens, with a special place where they could, when needed, insert a stone of the second kind and the oven would become hot. The AAAs used "normal" sources of fuel, but despised the "ignorant" BBBs and their stupid ovens.
Little by little social differences started to disappear. BBBs would go to school, find various jobs, get away from survival agriculture. And they got ashamed of their old traditions and would turn to "normal" sources of fuel as soon as they could afford for them. Also I suppose you could not use their ovens to have light at night or power a fridge, or use in a car. So they wanted gas, electricity, TVs, cell-phones, etc., etc. So they threw away their ovens and lived like the AAAs.
At some point a BBB with a scientific education realised they were losing a very precious knowledge, not just for the money they could spare, but for its scientific value. So he started to research on the stoves. Alas, all the BBBs he could find had already lost or destroyed their ovens. Somehow he heard about an oven that has not yet been destroyed (maybe belonging to some distant relative). But just when he arrived at the place where that oven was supposed to be, quite possibly the last one in existence, he found out that its very old owner had finally decided to renounce this "stupid" tradition, had ground the stones of both kind he still had and had thrown away the resulting sand.
No more BBB ovens. Sigh.....
Maybe rather than adding a comment, I should edit the question itself ?
Though I read this story in English, I do think about it in French. "Oven" is back-translated from my memory of "un four". But it might have been "un réchaud, un poêle, un fourneau, .... ", a cooker, a boiler, a stove, a burner, a heater....
