What blocks can you use to put under a beacon?

iron blocks, gold blocks, emerald blocks, and diamond blocks.

You can use Iron, Gold(I think), Diamond, and Emerald blocks.

From the wiki page, emphasis mine:

Activation

In order to activate a beacon, the beacon must adhere to the following requirements

• Beacons require an unobstructed view of the sky, though bedrock and transparent blocks (including water, lava, slime, and other beacon blocks) will not count as obstruction.
• The beacon is on top of a pyramid constructed from iron blocks, gold blocks, emerald blocks, and/or diamond blocks.

To activate a beacon, you need to make a pyramid of iron blocks, Diamond blocks, Gold blocks, or Emerald blocks [You can use all of them]. At the top in the middle of a 3 by 3 square put the beacon and then later it should glow.

This is what it should look like at any size:

But hey don't go away! [SPOILER!]:

With the new update you can also use Netherite.

I don't understand why you would use any pyramid material other than Iron. The effect depends on the amount of layers, not on the value of the material, so if Iron is the cheapest option, there's no reason to use anything else. Gold, Diamond, Emerald (since 1.14), and especially the new Netherite are very valuable and shouldn't be wasted on a beacon.

Gold can be used for:

1. Golden Carrots (best food in the game)

2. Golden Apples (amazing regeneration effect)

3. Crafting Netherite Ingots (best material in the game)

Diamond can be used for:

1. Diamond Tools and Weapons (2nd best material, can be turned into the best material)
2. Jukebox (useless but fun)

Emerald can be used for trading with Vilagers, their trades can actually be useful since 1.14

And Netherite is just straight up OP.