I agree with Jamey that this is likely magnesite.

The brown veining may be an enhancement produced by crazing with heating/cooling and dipping in a brown dye. The beads may be otherwise stabilized with resin to harden it. Some appear to be resin impregnated or lacquer coated. No one has done enough research to tell exactly what enhancements are involved. Dyed blue or green it is commonly sold at "white turquoise" or "chalk turquoise."

Eddy Thommason from Thommason stone in Albuquerque had the GIA test some with the results that the material is a "dyed carbonate" imitating turquoise.

If it is, in fact, Magnesium Carbonate (MgCO3), soaking a cracked bead in a dilute acid like white vinegar should produce bubbles of carbon dioxide.

details about magnesite at http://www.galleries.com/minerals/carbonat/magnesit/magnesit.htm

Amazonite, on the other hand, is a form of microcline, a Potassium Aluminum Silicate.

There is no reason these beads are not coming direct to Africa. Indian glass and stone beads are sold into Africa. Chinese fiber optic and coated glass beads are sold direct into Africa and the Middle East. The so called "Mecca beads" , an oval bead with random spray painted coatings in several colors are made in a factory that I've visited in China.

If this dealer visits you in Montana, please ask him to stop in to see me in Spokane an his way through. I'll be glad to show some samples of magnesite rough from China and a variety of other magnesite samples.