Help identifying unusual rock/gem please!

[peaceofmind]

Hi everyone:

I came across these two rocks at a gem and crystal shop, but unfortunately didn't catch their name. Despite their unique shape, i'm having quite a hard time identifying them. I've tried searching online but to no avail. Has anyone seen anything like these before? I know that they're both the same type of rock. Thanks in advance!

 

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[Moesia]

Try scratching them whit a steal knife, does it leave a grey mark or a scratch?

 

[Eu_citzen]

My first thought was concretions. But I'd go about as Moesia suggested.
I'd also try rubbing them on a unglazed piece of porcelain, what color do you get? (the latter is called streak test, BTW)

 

[DDancer]

I'll hazard to guess that the one with the white cap of crystals is a concretion and the crystals are probably calcite, a close up of the top and bottom on that one as well as the other would help. The three nodule one needs a couple of more angles and a streak test~ but looking at it it reminds me of a volcanic bolide. It looks as though it broke off something on the right side.