Crystals

A rock that vibrates. Piezos. You can have your piezo and eat it too (if it's salt).

Computers are a calculating rock.

LEDs are a rock that emits light.

Magic rock.

List

• https://youtu.be/MbPeYd1N1Xg - How to characterize crystal parameters. This video walks through turning a crystal into a strict electrical circuit, so you can adjust the frequency, or calculate what capacitance is needed. This subject matter is also covered in AoE, 3rd edition, section 7.1.6.
• https://www.statek.com/technical-papers/ - Some simple and good data sheets on quartz crystals. Recommended. Maybe someday they will put all of them online.
• https://www.youtube.com/watch?v=yYGwfVnGAdg - The Signal Path youtube channel. Comparing Crystals with Ring Oscillators. The ring oscillators (based on 74 type logic) are much less accurate.