Chalcedony and agates are quartz and have a hardness of 6, which is just a tad less than jadeite, but they are much less tough and thus are more brittle. It's the toughness not the hardness which makes jadeite durable. All that said, I've carved agate with my tremel tool using diamond tipped points, and it is amazing how tough it is! I'm sure if you gave quarts a strong impact though, it would shatter.
BTW, last night my B jade lavender bangle fell from my dresser onto the bouncy wooden floor....and shattered into 8 pieces! Talk about the compromised strength of B jade, sheesh. I am a little sad because though I stopped loving it after discovering it was in fact B jade, I thought of it as an expensive accessory. Oh Well.
Sikat- Jadebuddhashop looks legit, their prices are similar to JOJO, but I don't know anything else. I've wondered the same thing.