Much like a heart is used a lot for health and lives in old retro games...
...what would be a good icon to use for Energy, Mana and/or Magic points?
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I don't know of any particular icons, but I think the most important thing to keep in mind is that players associate the colour blue with magic. Thus, I don't think the icon actually matters, as long as you follow the basic convention of red for health, blue for magic and green (or the less-used yellow) for a third metric (usually stamina, but can be other things like in the case of Eternal Darkness: Sanity's Requiem where green stood for sanity).
Typical shapes include orbs, crystals, potions or runes.
Hope this helps!