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Quite some time ago I did a whole big blog post on the subject of books and their manufacture. As a writer, this is of natural interest (though it took a long time for me to look into it). In the course of my writing, however, I came across a man who needed to write, but for different purposes. It's a tool. He doesn't want to preserve his writing for the ages. That man is an engineer and an educator. I could always make plenty of ink and paper available to him, but it felt more characterful and evocative to hear the chalk squeaking across the slate. Then I realized, "I need to know where he got those things."
Slate is a metamorphosed shale or mudstone (these are rocks, people). It is relatively fragile and naturally forms sheets. Because of the latter quality, slate was used as roofing material long before it became a ground for writing. Fun fact: the easy way to tell the difference between shale and slate is by chewing them (slate has a creamy consistency). The first European reference to writing slates come from Chaucer's Treatise on the Astrolabe (ca 1391). Similar "boards" were noted in India in the 11th Century. Slates were generally double sided with a wooden or cloth frame (to protect the fragile slate) and might be hinged to include two or more "pages." Lines were etched into some slates to make it easier for writing practice. They were used by schoolchildren, merchants, and gamblers alike (though hopefully for somewhat different reasons). Large scale blackboards were not used until much later (ca 16th C), due to the weight and fragility of the material.
Chalk is what I remember using on the "blackboards" in my youth. Then again, the "blackboard chalk" we used may have been made from gypsum. Anyway, chalk forms in deep marine conditions from a gradual accretion of calcite (calcium carbonate) plates. In nature, it is normally associated with clays and tends to stand out as the clays degrade around it (see: the white cliffs of Dover). Chalk is nice and crumbly, for writing on rough surfaces, but kind of a mess. What was commonly used on slate tablets was the "slate pencil," which was made of soapstone or a softer slate, with a wooden handle. These made less mess, but the sound was apparently worse than nails on a chalkboard (though my parents' dinner plates made roughly the same sound when scratched by a fork or knife, so the squeal tended to make me hungry). Later evolutions in slate technology tried to take away the squeak. This isn't to say that chalk was not used, simply that there was competition.
Cool, I'd never heard of slate pencils before today, but they make perfect sense. The stresses of metamorphism (we need to form the slate) are not uniform, so naturally you find different hardness (and color) materials relatively close together. Ready made writing tools. In contrast, chalk seems an unlikely choice. Mudstones originate in freshwater, while chalks are from saltwater sources, so the odds of having them occur near to each other are not so great. However, as we've seen, geology can do some funky things (given enough time). Wales has a great source of slate and there is plenty of chalk in England, so there you go. The personal slate was an incredible leap forward in the classroom, allowing extensive practice in writing and arithmetic without the extreme expense of pen and ink. As the use of blackboards slowly declines, it makes me wonder how teachers punish bad students. I can still smell the chalk dust.
Wiki - http://en.wikipedia.org/wiki/Slate_%28writing%29
blackboard history - http://education.cu-portland.edu/blog/reference-material/the-history-of-the-classroom-blackboard/
slates and slate pencils - http://pastperiodspress.com/2012/08/31/slates-slate-pencils/
pencils - http://www.officemuseum.com/pencil_history.htm
Detailed History (related material begins on pg 33) - http://etd.lib.umt.edu/theses/available/etd-06062008-182026/unrestricted/Swords_Molly_Thesis_PDF.pdf
Indian History - http://www.archive.org/stream/alberunisindiaac01biru#page/162/mode/2up
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