How do you get all the gems, and what do they do?

After doing all 4 boards in normal mode, I got a red gem. It placed itself at the top of the star.

I guess there are others. But I also can't figure out what are they supposed to do.

So how do you get all the gems, and what do they do?

According to IGN there are 5 different gems, each unlocked in a different way.

At the start of Super Mario Party, you're asked to unlock five different gems. Here's what you need to do to unlock each one:

• Gem of Tenacity - Play all four boards in the standard Mario Party mode.
• Gem of Spirit - Complete the Sound Stage mode in Hard difficulty.
• Gem of Courage - Play through every route in River Survival.
• Gem of Love - Play all four boards in the Partner Party mode.
• Gem of Passion - Complete every world in Challenge Road.

And as reward:

After you unlock all the gems, you'll be able to watch the final cutscene.