Fill boiling water bath canner with hot water. Place 7-1/2 pint (250 ml) preserving jars in canner over high heat (to sterilize). Place metal snap lids in boiling water. Boil 5 min to soften sealing compound. Keep hot until ready to use.
Prepare chokecherries - wash, stem, and crush fruit. Simmer with 1 cup (250 ml) water until tender. Put through sieve or food mill to remove stones. Measure 31/2 cups (875 ml) fruit and 1/2 cup (125 ml) lemon juice. Place fruit in a 5 qt (5 L) saucepan or kettle. Add 1/4 tsp (1 ml) butter to reduce foaming. Add pectin crystals to fruit and stir well. Place over high heat and bring to a boil, stirring constantly to avoid scorching.
Remove from heat, add sugar, and stire until all sugar is dissolved. Return to heat, continue stirring, and bring to a full rolling boil that cannot be stirred down. Boil hard for 1 min, stirring constantly. Remove from heat and skim off foam. Stir and skim for 5 min to cool slightly to reduce floating fruit.
Ladle into hot sterile jar. Leave 1/2 inch (1 cm) head space. Clean jar rim. Center metal snap lid. Apply screw band until finger tip tight. Place in canner. Repeat with remaining jars. Adjust boiling water level to 1 inch (2.5 cm) above jar tops. Cover canner. Begin process time when water returns to a boil. Process 10 min in boiling water bath. Remove from canner. Set upright and spaced apart out of drafts to cool. Cool 24 hours. Test for seal (sealed lids curve downward in center). Remove screw bands. Wipe jars, label, and date. Store in a cool, dark location. Yield: 7-1/2 pint (250 ml) jars.