First appearance: Act 2

Queen Beryl gives Jadeite the brown crystal on the left at the beginning of act 2. It has the power to turn inanimate objects into monsters that look like they're straight out of Power Rangers. He uses a different crystal to do the same thing at the end of act 2, when he creates the monster that appears in act 3. Similarly, Nephrite uses one of these crystals in act 4.

I had previously thought that Jadeite and Nephrite had only one of these crystals each, but as Adric pointed out to me, the crystal Jadeite uses at the end of act 2 is a different colour to the one he uses earlier on. It seems that each of these crystals can only be used once.

Note: These crystals don't have a name as far as I'm aware.

Anime/Manga Comparison:
The anime has an item called the Black Crystal, which Nephrite and then Zoisite use to do various monster-related things. That may be the inspiration for these items. The manga doesn't have an equivalent of this. The manga did have crystals that were related to the Four Heavenly Kings (made of their namesake minerals), but these crystals had nothing in common with the ones used to create monsters in PGSM.