I think there is a big difference between sharp and practical. Because stone is harder than steel it should take a sharper edge but because it is more brittle than steel that edge would be more delicate - probably more delicate than is needed to make a practical edge.
In the stone age, that's what people used to cut things so sharpening stones has been done for a very, very long time as we all know. I've seen them made from flint and obsidian and even found a couple of obsidian ones once myself in the Mt. Jefferson primitive area in Oregon. The few stone age arrowheads I've seen are quite sharp and effective but wouldn't be very durable as a knife. I assume those people used stone for knives also but there is no doubt metals completely replaced them when the technology became available.
My experience with modern ceramic blades is similar. If a ceramic blade is ground to an angle as sharp as steel or sharper, the edge will be delicate and, in my opinion, too delicate to be practical. Steel is still the way to go. Sharp and practical.
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