How do I match 15 star gems in Bejeweled Blitz?

[JazzPurr]

Last challenge in Gem Explorers Pro
Which power gem is best to use?

 

[amelia93]

It's soooo easy to make a star gem

 

[zaida_wh]

Last challenge in Gem Explorers Pro
Which power gem is best to use?

use the CORALITE (purple rare gems)