Regarding the gems: Each one has a LEAD stat, which is weighted in a 60/40 which becomes more evident with higher level items and a Grand gem variation. The lead stat in all cases is the FIRST stat listed in the description of the gem/recipe.
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All of the recipes available during the event can only be acquired during the event, and they will eventually go away after the event is over. They are finite, but vendor for gold after the fact. The plan is to keep the recipes online for another week or so after Tarlok's Wrath comes to a close to allow players to wrap up any final crafts before they become unavailable. All gems and chests that exist at that time will be the last ones available for use.
Also note that any winter tokens not used will be removed at the end of the event, so don't let them go to waste!