Wash the oranges. Using a microplane or similar grater, zest two of the oranges, being careful not to dig down into the white.
Put the zest in a large pot.
Peel all the oranges, discard the peel. Take the oranges apart and get rid of all white pith and tough membranes -- they won't get any more interesting on your toast.
Take the cleaned orange flesh, chop up into small pieces and add them (and any juice on the chopping board) to the large pot.
Chop up the rhubarb into 2 to 3 cm pieces (1 inch), add to pot.
Add water to the pot.
Put pot on a high heat burner, then when it starts to boil, cover, and lower to a simmer for 20 minutes. Stir occasionally.
Remove pot from heat.
Stir the calcium water into the pot.
Mix the sugar and pectin together, set aside. (If using a liquid sweetener such as liquid stevia or honey, mix the lemon juice and pectin powder together quickly). If using sugar, add the lemon juice instead now to the pot.
Stir the pectin mixture into the pot. If using a liquid sweetener, add it now as well to the pot.
Put pot back on the burner and bring back to a full boil.
Let boil for 1 to 2 minutes, stirring constantly, then remove from heat.
Ladle into quarter-litre (½ US pint / 8 oz) jars.
Leave 1 cm (¼ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process for 10 minutes; increase time as needed for your altitude.
Best after at least a week of jar time for the flavours to meld.