This is a pretty handy chart if you are considering purchasing a litho stone, or even picking one up to move around. For example, a 16 x 12 inch stone weights 43 pounds per inch of thickness. This was pasted onto the inner face of the front board of Printing and its Accessories from Massy College. Another reminder that many printed books contain unique, owner added paratextual information.
Peachey Conservation 
Have you misread the chart? It appears to say that a 12 x 16 has an “average weight in lbs” of 43 lbs. Not pounds per inch. Limestone weighs in the neighborhood of 150 lbs per cubic foot, so a block 12 x 16 x 3 (one third of a cubic foot) would weigh about 50 lbs or so.
You are right, Thanks! I don’t think I’d be able to move a 16 x 12 stone if it were three inches thick and around 150 lbs. Duh.
There seems to be a lot of variation in the weight of litho stones. This site:
http://www.polymetaal.nl/siteUK/shopukwork/en-gb/dept_38.html
has a list of several hundred stones for sale, with weights and dimensions. The first and third at present are both “hard yellow” grade, roughly 6″ x 10″ x 2.2″, but the first is roughly 13 pounds and the third roughly 20. The table’s prediction (halving the size of a 12″ x 10″ stone) would be 12 pounds. I browsed a bit in the list, not very systematically, and throughout there seems to be a similar level of variation but very rough correspondence with the table.
I draw your attention to the definitions of the seven color grades of litho stones. I had previously distinguished a creamy color that is preferable because it is gentler to the edge of a paring knife, and a cold grey color that is harsher, but I came nowhere near seven colors. I suspect that the creamy stones I like are grade #5, “Hard yellow stones,” less hard and less dense than the “blue” and “grey” stones but more uniform in density than the “soft yellow stones;” but I am open to correction. If I am right, there is the pleasant conclusion that the most desirable grade for binding is not the most desirable grade for lithography; this goes nicely with the dichotomy that thin, worn-out stones are best for binders, whereas thick, new stones are best for printers.
Thanks for this link, I had no idea there were such a range of colors. I’m also curious what these colors actually look like, since most (all?) of the stones I’ve seen are roughly 18% grey.
I did weigh my grey stone: 16 x 12 x 3 and 55 lbs.
In practice, I doubt the slight difference in hardness is all that important, since it it rare that one digs into the stone with your knife.
It’s not digging into the stone that’s the problem; rather, it’s the slight gradual dulling from the knife rubbing during normal paring. Imagine the difference between using a Japanese “gold” sharpening stone as a paring surface, and using an 800-grit stone. The difference between litho stones is not that extreme, but it feels like that: a shade of grittiness under the blade. The difference is small, just as the color differences are subtle, and I’m not at all sure I could now tell which kind a stone was without one of the other kind to compare; but back when I was frequently paring on a variety of stones I was convinced that the difference in harshness was there, and that knives dulled appreciably faster on the harsher stones. And, of course, those French non-litho purpose-made paring stones that look like inch-thick granite are abrasive enough that French-style binders can use them for touching up their edges every few minutes. I’ve never been able to get that trick to work on a litho stone. Glass is said to be harsher than litho stone. In passing, there used to be a local cheap trade bindery where the paring surface was the armored-glass front window out of a WWII fighter plane, clearly identifiable by the trapezoidal shape and by the thickness, something more than 2″ as best I remember.
If the chart’s “average” stone were 2.3″ or 2.4″ thick it would be right in line with the weight of yours at 3″ thick. I can’t weigh mine, but my 13″ x 17″ stone is 2-1/2″ thick, and my big 20″ x 16″ is 3-1/2″ thick. I have two others intermediate in size and 2-3/4″ thick, but these are buried too deeply to get the other measurements.
I usually use glass to pare on, and really haven’t noticed an appreciable difference in how sharp the knife stays as compared to a litho stone. But I don’t let it scrape on the surface. As a party trick last year at North Bennett Street School, after demoing the making of a knife from a hacksaw blade, I edge pared a small piece of goat all of the way around twice in one long piece, on top of a piece of tissue on top of the litho stone. No fibers in the tissue were cut into.