The best mandarins for jam are full-flavored and juicy, but they don't need to be particularly sweet. In any case, you should employ your tastebuds to find the optimum balance of sugar for your batch of fruit. While you can certainly use any variety of mandarin to make this, if you're using something particularly small, throw in a couple extra to make up for the higher proportion of peel (FYI, the ones I based this recipe on were about 1/4 pound each). I imagine you could also employ this technique for just about any kind of citrus fruit - meyer lemons and grapefruit come to mind, for instance - though naturally the quantities of sugar and lemon would probably need some adjustment.
Yield: about 1 quart; recipe can be easily scaled up
1 kilo (generous 2 lbs) mandarins, any variety: clementines, tangerines, satsumas, etc., preferably organic
about 500-600 grams (2 1/2-3 cups) sugar
juice of 2-3 lemons
Wash 2-3 of your mandarins and zest them, carefully avoiding the white pith underneath. You should have about a tablespoon. Peel all of your fruit, removing as much of the pith and filaments as possible. Working over a bowl to catch the juices, cut each mandarin in half around its equator, and pick out any seeds. Place the halves along with any juice they've expelled in a food processor and process for about a minute, until you have a more or less smooth puree.
Combine the mandarin puree, tablespoon of zest, sugar and lemon juice in a large, heavy saucepan. Bring to a boil over medium-high heat, then reduce the heat slightly. Allow the mixture to boil, stirring occasionally, until it sets, about 30 minutes. To test the set, place a small saucer in the freezer for a couple of minutes, then drizzle a teaspoon of hot jam on it. Allow to cool, then run your finger through it. If it holds the trough, it's set. Also taste for sweetness at this point - if you think it could use more, add a bit more sugar and cook another minute or two; do the same with lemon juice if the acidity needs some perking up.
Pour into hot, sterilized jars, seal tightly and turn upside down until cool. Or use your tried-and-true canning method. Or simply keep in a closed container in the fridge for up a month.