All about, but cannot be seen,
Can be captured, cannot be held,....
Answer: 'wind'
Three lives have I.
Gentle enough to soothe the skin
Answer: 'water'
I am always hungry,
I must always be fed
Answer: 'fire'
Bright as diamonds,
Loud as thunder
Answer: 'waterfall'
http://www.gamefaqs.com/pc/564770-conquests-of-camelot-the-search-for-the-grail/faqs/44699
Voiceless it cries,
Wingless flutters,
Toothless bites,
Mouthless mutters.
Answer: wind
What flies forever and rests never?
Answer: wind
What moves before the sun but does not leave a shadow?
Answer: wind
What can you hold without ever touching or using your hands or feet?
Answer: your breath
http://www.lisashea.com/hobbies/dnd/riddles/wind.html
As for earth... this seems to be tricky indeed. I got some things related... Perhaps you can alter the riddles a little or maybe it'll inspire you to create the ones you need.
The more you take, the more you leave behind
Answer: Footprints
Born of Earth, but with none of its strength.
Molded by Flame, but with none of its power
Shaped by Wind, and with all of its clarity
Answer: Glass (I added this because it seems to envelope 3 of your elements. You could add in the end something related to water, e.g. Diverse as Water, with none of its traits)
In we go
out we go
all around
and in a row
always, always steady flow
when we'll stop you'll never know
in we go
out we go
Answer: Tides (maybe you can alter this into an earth riddle?)
I am the third from a sparkle bright,
I thrive throughout the day and night,
Deep in the path of a cows white drink,
I've had thousands of millions of years to think.
But one of my creatures is killing me,
And so the question I ask to thee,
Is who. Am. I?
Answer: Earth (the planet - but you'd have to either leave out the first sentence or alter it)
What has roots as nobody sees,
Is taller than trees,
Up, up it goes,
And yet never grows?
Answer: A mountain
I took these riddles from the following page (if you haven't been there yet you really should have a look)
http://www.thievesguild.cc/riddles?p=1
Good luck man, hope it helped a bit.