gems from cards/backgrounds/emoticons

100 gems are usually from the special holiday items and sale stuffs from Steam.

http://steamtradingcards.wikia.com/wiki/Gems_by_Game
All the Valve games that have emoticons convert to 100 gems.
Also, the holiday items are 100 gems too, mentioned above.

Edit: Sorry, I lied. Portal is a Valve game and those items give 80 gems.
Here's some off the top of my head:
Dota 2, Team Fortress 2, Counter Strike: Global Offensive

From what I've seen, only Valve game backgrounds and emotes give 100 gems.

Trading cards and emoticons, in my experience, are usually worth a LOT less than 100 gems, I've generally been lucky to get more than 20 gems for emoticons whereas I either sell or craft with cards...mind you, the most I've got from a background is 80 gems so maybe I'm doing it wrong...

thanks for the info guys, have a good one :)

hahaaa....i did this trick months ago
i bought 100 background / emoticon
then turn it into gems
XD

but i cant do this again now, because all price already fix?
or maybe steam market have a bot to buy all item if price is below in specific price?