Our intrepid wizard protagonist realizes there is only one of him and an army of baddies. He has a band of some 50 people with him, but only a dozen have combat training and he is the only spellcaster. The guards are working on tactics, wooden spikes, choke points, and etc. Our wizard wants to make weapons to help.
Deciding the best chance of survival lies in keeping enemies at a distance, our wizard looks for how to do that while also overcoming the numerical odds against them. Knowing he doesn't have enough spell slots/mana/mojo to fire the requisite number of fireballs and something like arrows won't be enough to offset the numbers, he settles on grenades.
His magic grenades use runes to hold magic. When the runes are damaged, the magic stored within explodes. This means his grenades need to break (or preferably shatter) on impact.
He and his band are in a mountain forest with no cities available. He has three months to stockpile grenades, knowing that when the winter ends, his enemies will be upon him. He has access to metal tools, brick, stone, wood, animals, snow, and everything else you would expect in a mountain forest with medieval technology. Some magic to aid in production, but no alchemy or other easy solutions.
What real-world material can he use which can be both thrown easily (like grenades) and will shatter, or at least break, on impact?
Is there a type of stone that fits the bill? Is slate brittle enough, for example? Can the brittle stone be shaped into a sphere for easier throwing? Can hollow bricks be made for this purpose? Is there a form of primitive glass he could make, or another material that would work?