As Tony stated, slate is indeed a metamorphic rock which began its life as a sedimentary rock called shale. During metamorphism, the elevated temperature and pressure turn the clay minerals in the shale to very small flakes of mica which all tend to lie in the same plane, which gives the slate the ability to split into flat sheets.
"Brunstone" as many have stated, is not slate but a fine-grained sandstone, a sedimentary rock. It may contain a little mica, which is a flexible mineral, but brunstone is mostly quartz and feldspar and this makes Brunstone less flexible/more brittle than slate. I have looked pretty closely at the Brunstone in my GC1 and although it is sandstone, it is either slightly metamorphosed or the grains are very well cemented together, most likely by quartz. Brunstone is a hard, tough rock, not crumbly at all.
"Bluestone" is not a geologic term, but rather a trade name for a wide variety of commercially sold rock that have blue or gray tints. This happens all the time: your "granite" counter tops may or may not be granite. In the counter-top trade, just about any rock that is not marble is called "granite". I've seen "bluestone" that is limestone and "bluestone" that is sandstone.
There is a grayish-blue commercial sandstone that is quarried in Pennsylvania that is called "bluestone". This rock is part of a ~380 million year old group of sandstones that are exposed in a band of rock that runs through eastern PA and is part of the ancient Catskill Delta. Although some of the Catskill Delta rocks are blue/gray, many are reddish brown, just like my Brunstone.
So, here's my theory: Brunstone = Pennsylvania "Bluestone". The quarrymen probably call ALL of the Catskill Delta rocks that they mine "Bluestone", because that is the trade name. Doesn't matter if the rocks are reddish-brown or truly blue/gray, to them it's all "bluestone". I suspect that some people at Brunswick would occasionally call the rock in the GC1s "bluestone" because that is the trade name of the rock rather than the made-up name "brunstone".