Chalk wasn't and isn't used as a writing tool. Gypsum is.
"
Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm long." (Look up "chalk", sub-section "uses" in wikipedia.)
--Since gypsum has to be compressed to make "chalk", it might not be appropriate (especially since Toady has a target period of pre-1500ish, IIRC), though I will make a note about gypsum in the first post.
But I think writing implements deserve their own thread.
And there are several threads dealing with all sorts of writing systems and implications, the purpose of this one is to narrow done stone/mineral/etc. uses more generally. Though I'm sure some discussion of other systems (i.e. writing, food flavorings, etc.) would probably help in suggesting those uses.
It takes hours to make a chert dart, as apposed to minutes for obsidian or metal.
Perhaps, but if you don't have access to obsidian or metal but do have access to chert, it would probably be better than nothing.
You can decorate with jet already, and jet in real life acts more like a rock then a gem anyway.
You can decorate with "stone", but you can't specifically decorate with jet unless you do something like burrows or turn off all other stone in the economic stone list, nor can you cut jet if you want a black gem for whatever reason (think mourning, or other serious occasions).
Why roof?
Because before modern shingles were developed, humans used slate primarily in roofs, blackboards and slates. There are a lot of derelict slate quarries in Pennsylvania that went out of business when slate roofs were replaced by other, newly-cheapened building materials.
Graphite wasn't even discovered to mark intill 1500, and only on sheep.
Which is roughly the cut-off point Toady has stated that he would prefer to maintain (though things like casts, etc. are clearly much more modern).
Most of the things you're calling clay are additives or are rocks found in clay.
The only two things I refer to as "clay" are kaolinite, and clay soil layers. Kaolinite is a powdery mineral of approximately the same hardness as natural chalk, and is considered to be a primary clay since it is not formed from weathered and eroded material. Clay soil is composed of weathered and eroded material, and therefore is considered to be a secondary clay. Both types of clay have maximum grain sizes of less than 2 μm (microns). The other minerals that I refer to as being used to make pottery, are used in addition to the clay source (such as the feldspars in ceramics). (Look up "clay" in wikipedia for a quick run-down). I did clarify the appropriate bits of the first post, however.
No, peat, baked and dried, will burn for 30 minutes to 3 hours, and it can get steel to a workable heat.
Adjusted.