The Seasonal Gem can be mined from Seasonal Ore.  Information on its spawning can be found under the Ores Tab.  With this gem, you can craft Winter, Spring, Summer & Autumn Gems.

Winter Holidays -

Valentines Day: You can currently craft a Heart Gem.  With that gem, you can craft the unbreakable Heart Sword, and it does 10 Damage.  The Heart Candy provides 5 food and has low saturation.