concrétions de magnésie de Nouvelle Calédonie

12:34 Jun 7, 2010 

looks quite hopeful. I've asked for clarification from the client.

23:02 Jun 3, 2010 

The modern name for magnesia is periclase (http://en.wikipedia.org/wiki/Periclase). But as Chris suggested, magnésie could be a typo for magnésite, magnesite in English. I'd say this is the more plausible option: polished magnesite was (and is) used as a gemstone bead, either as found in nature or polished afterwards.
(A word of caution regarding your specimen image: one mineral can have many different appearances.)

20:43 Jun 3, 2010 

for your input. As I mentioned to Chris, I have now seen a picture of the object in question, it is a lumpy rock looking like a cloud. Not translucent, no rainbow effects. I have no knowledge of geology but it looks like a fairly porous volcanic rock - could that be right? I can't post the picture I'm afraid. It is part of a museum exhibit, I cannot risk an incorrect description. I had imagined there might have been a more common name. Thank you all wholeheartedly for sharing your thoughts on this.

20:26 Jun 3, 2010 

Chris, good question. Only the asker can decide.

20:21 Jun 3, 2010 

does this get us any closer? The concretions referred to are manganese, not magnesium, but it does appear from the paper that New Caledonia is rich in magnesium carbonate, which is a crystalline material capable of refracting light to produce the characteristic rainbow colours mentioned in the text.

19:50 Jun 3, 2010 

16:17 Jun 3, 2010 

It is possible that, in this context, "magnésie" refers to the crystalline form of magnesium carbonate. The more usual meaning of magnesia is of course the oxide, ie MgO, but my basic geology suggests to me that this mineral is too soft to form concretions, and also liable to evolve into the hydroxide under the action of water. See ref. below: http://en.wikipedia.org/wiki/Magnesium for the various compounds of Mg.