http://www.hooverandstrong.com/articles/?id=28
Thanks Julie, try this one instead - they've archived the page.
http://www.hooverandstrong.com/blog/archives/53/
PS: I don't even remember posting this link!
This link is broken :(
Sorted! Thanks Julie ;)
The link moved again: http://technical-articles.hooverandstrong.com/wordpress/opalino-marbleized-beads/