Improved Familiar - Crysmals a go?

[Ethereal Gears]

The feat Improved Familiar seems to suggest an arcane spellcaster of 5th level or higher can gain any Small creature with the elemental subtype as an improved familiar. Is this correct? If not, how do I know which elementals are eligible? Is it only those with 'elemental' in their name, or would I be able to get a crysmal as a familiar, for instance? I wants my shiny glittery gem bug!

[Bob Bob Bob]

To the best of my knowledge, it has to be an "X elemental", where X is air, water, fire, etc. So you could get a lightning or ice elemental (added in bestiary 2), but not anything with the elemental subtype that's small in size. Since the listing is "Elemental, Small (any type)" there probably have to be "types" of whatever you're picking, and there are no other Crysmals, just the one species. There are six? official Paizo elementals (and plenty of 3PP ones). And since it requires types of Elementals it probably requires they have Elemental in the name as all of the other entries are for the name of the creature, not the type or subtype.