riddle - Add two more and you'll have me - Clue Thirteen

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The light shuts off, and you are left in the dark - for a moment. Then you notice a glow coming from the tiled floor. You peer down to have a better look. In a 3x2 space, the six tiles say:

I'm essential to life
But maybe not quite.

This is number thirteen

But add two more and you'll have me.

I'm not bright
But I still have some light.

I don't have a lamb
But I am inside one.

If you follow my lure
You may need a cure.

I am red and white, but not blue.
Instead, I'm black and violet - scarlet too.

Each of letters is glowing slightly white, with a glass covering above the letters. Next to them, there is that familiar keyboard for the answer. You reach over it and type...

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Answer

Are you

Phosphorus?

I'm essential to life
But maybe not quite.

Phosphates are part of DNA and such

This is number thirteen
But add two more and you'll have me.

Phosphorus is 15 on the Periodic Table

I'm not bright
But I still have some light.

Phosphorescence is a glow, but usually not bright.

I don't have a lamb
But I am inside one.

Uraguay apparently bred glowing sheep

If you follow my lure
You may need a cure.

Phosphorus is poisonous

I am red and white, but not blue.
Instead, I'm black and violet - scarlet too.

Red and White are the most common Phosphorus allotropes, but violet black and scarlet also exist

Another hint:

The keys are glowing