Jam No Gel

Put the whole thing in a sieve and drain off the syrup. Bring the syrup up to 221 degrees. I would add something for pectin to the syrup, since cherries are low-pectin. Apple jelly would work. When the syrup reaches temperature, add the cherries back in and simmer for 5-10 minutes. Test to see if you've got a gel. If not, simmer a bit longer. You should get a gel this way, even if if it a little soft.

I believe that cherries have a low pectin content and usually require some pectin--or addition of a fruit with high pectin content such as apples. If you are not experienced in making jams and jellies, it would be a good idea to follow the recipe until you get the hang of preserves and have an understanding of how you can successfully improvise. You could depart from the recipe in flavor additions, perhaps, such as adding herbs or complementary flavorings, but try to stick to the basics of the recipe.

I'm not familiar with Leibovitz's recipe, but something described as "no recipe" sounds pretty open-ended; I would have recommended that you try something a little more structured for your first foray into canning. What volume of fruit (after the first 20 minutes of cooking) did you have, compared to your 2.5 cups of sugar? It should have been around 2/3-3/4 of a cup of sugar per cup of cooked fruit.

Assuming that it was, did you bring the jam to 12 degrees over boiling (220 at sea level)? That's your set point, so if you didn't get it that hot or you're not sure how hot it got, reheat it and bring it up to that temperature, stirring regularly to keep the sugar from sticking. If there's sufficient sugar to the volume of fruit, that should make it set.

I cooked it for about 20 minutes without the sugar/lemon and then another 15-20 with. I didn't want the sugar to caramelize.

How long did you let it cook? It may just need to be cooked longer. Also, I sometimes add chopped up apple for the natural pectin if I'm not following a recipe (and not planning on canning -- then I follow recipes to the letter).