What are the effects of no Diamond monopoly?

Lets say for whatever reason, the giant Diamond fields of South Africa are exploited by many competitive firms.
There is no Diamond monopoly and no big marketing campaign that turns them into the most wanted gem.
(some short interesting points: http://www.wanderings.net/notebook/Main/10ReasonsWhyYouShouldNeverBuyDiamonds)

Now, how cheap would diamonds be? And the most important question, how would affordable diamonds change industry, consumer goods, basically the non luxury market?
 
And used industrially on a massive scale, and even as a supplement to Coal at times.

I doubt it would be worth less than coal. Coal is dirt cheap relatively speaking and there aren't that many diamonds. Their use in industrially would certainly skyrocket though.
 
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