I absolutely love sour cherry jam. To me tart or sour cherries are perfect for two things - pies and jam. This is a simple and delicious, slightly tart jam.
In a large, deep, heavy-bottomed pot, combine cherries and lemon juice. Stir in pectin until dissolved. Bring to a full boil over high heat, stirring constantly. Add sugar in a steady stream, stirring constantly.
Return to a full boil, stirring constantly to dissolve sugar. Boil hard for 1 minute. Remove from heat and skim off any foam. Stir for 5 to 8 minutes to prevent floating fruit.
Ladle into sterilized jars to within 1/4 inch (0.5 cm) of rim; wipe rims. Apply prepared lids and rings; tighten rings just until fingertip-tight. Process jars in a boiling water canner for 10 minutes.
Transfer jars to a towel-lined surface and let rest at room temperature until set. Check seals; refrigerate any unsealed jars for up to 3 weeks.