Wash, peel, and slice the peaches. Blanch in boiling water for 30 seconds, then plunge into ice water to easily remove skins.

In a large saucepan, combine the sliced peaches, sugar, lemon juice, and cinnamon (if using). Stir well to coat the peaches.

Bring the mixture to a rolling boil over medium-high heat, stirring frequently to prevent sticking.

Reduce heat to medium-low and simmer, stirring occasionally, for 15-20 minutes, or until the jam thickens. It should coat the back of a spoon.

While the jam cooks, sterilize jars and lids by boiling them in water for 10 minutes. Let air dry.

Ladle the hot jam into the sterilized jars, leaving 1/4 inch headspace. Wipe rims clean.

Seal jars with lids and rings. Process in a boiling water bath for 10 minutes for long-term storage (optional).

Cool completely. Check seals; lids should be concave. Store in a cool, dark place.