Diffused 5mm Slow Fade Flashing RGB LED on CR2032 Battery

[PipX]

I'm doing a project with tabletop models where I want to illuminate some crystals with LEDs. If I'm using a CR2032 battery with 3V, 235 mAH, do I need to add a resistor in series with a single slow flashing LED?

[adafruit_support_bill]

Not really. A resistor will give you a longer runtime, but it will work without one:
http://www.evilmadscientist.com/2009/so ... -throwies/

[PipX]

Ah that's what I thought. I noticed though that on similar LEDs some of the colors (I think Red and White) show that they draw only around 1.6-1.8V, would using the 3V cell burn these out?

[Zener]

You are talking about the Vf of the LED which is how much voltage will be measured across it when it is on. Generally when you say something "draws" this or that we are talking about current not voltage. Nitpicker, nitpicker..

The Vf of an LED varies with color and the chemistry can have an affect also. Red are typically around 1.9 - 2, green 2.3 - 3 depending on type and blues 3v and up.