King Of Poisons and Poison Of Kings

I am the King of Poisons, and the Poison of Kings
To death, my foul vemon sings

I form myself colorless, so hard to be seen
My rhombohedral shape, makes me quite keen

In nature I am discovered in the ground
In water I dissolve, naught to be found

I am the deadly metal, strengthening my brothers, Tin and Copper
And quite a favorite among that Victorian Shopper

My grand number, is that of thirty three
While used, in treating the tree

With chalk and vinegar, I beautify
Yet in your drink, Life, I do deny

When used to keep your plants safe, I am quite a charm
Yet hiding in your well, I will only do harm

I was discovered by the man called "the Great"
Nearly Eight-Hundred years ago to date

So ask yourself, I do decree,
"Do you, truly know me?"