recently i pondered this, and this is my "homemade" version that i'd like to try...
use a small circle (or any shape- square, diamond etc.) cutter or the back of an icing tip to cut a circle out of fondant or gumpaste. use another smaller round cutter or a dowel and push it in middle to make an impression that is a little deeper than the edge. paint the outer circle with gold or silver luster dust and extract. then paint the inside circle with colored petal dust, powdered color, liquid or gel color. allow it to dry. then put a dot of clear piping gel over the middle colored part...i think it would give a transparent jewel effect.
you could also try a sugar recipe (like making hard candy) and buy a mold ...
hand mold your own out of fondant or gumpaste, dry, paint them with color, allow to dry and then give a slight dry dust of super pearl or ultra white luster dust, maybe pixie dust....to give the sparkle.
i think there is place called jewel of denial? on the web that sells premade sugar gems.
they make molds I've seen them.........and then you could just make some cooked sugar and color it and pour and when they're hard pull em' out.....I haven't seen the cake you're talking about but I'm assuming these were realistic looking gems??? here is a site that sells them
http://www.thejewelsofdenial.com/catalog.php?item=115&catid=12&ret=catalog.php%3Fcategory%3D12
here is a mold.....expensive though
http://www.jbprince.com/index.asp?PageAction=VIEWPROD&ProdID=3627
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