How do I get more Crystal?

[Kittenpox]

As the title asks - how do I acquire more Crystals?
I'm aiming to make another Sparkly Prism, to gather more Sparkles, but don't have the raw materials to do so, and there's no clearly visible way to gather more at the moment. :-s

[isaacbee]

Use fireworks to dig underground for crystals. If there aren't any currently visible, you can use torches to help you explore further underground.

[Kittenpox]

Thankyou