Stir apricots, lemon juice, and sugar together in a large pot over medium heat; add butter to reduce foaming. Bring apricot mixture to a rolling boil, stirring constantly. Quickly stir in pectin; return to a full boil until pectin is dissolved, 1 minute. Remove from heat and skim off any foam with a metal spoon.
Sterilize jars and lids in boiling water for at least 5 minutes. Pack apricot jam into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil, then carefully lower the jars into the pot using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process for 5 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, for at least 24 hours. Press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). It may take up to 2 weeks for jam to set. Store in a cool, dark area.