Easy peasy riddle

The answer is

Metal

Known since the ancient times

People have been using metals since ancient times.

I'm sort of a chameleon

Metals of different sorts and colors are available (A chameleon can change its colors).

Sometimes I am ugly

Raw metal, or rusted iron is ugly

And sometimes worth a million

Gold, platinum etc are worth millions.

This value, however, is nothing compared To what I can do if not handled with care

Metal machinery, if mishandled, can cause destruction of life and property.

You use me to leave a mark on a tree

An axe can be used to leave a mark on a tree.

Or to free life of impurity

Surgical instruments are made of metals, which can free someone's life of impurity (disease, germ)

My family is huge, I can't say I'm lonely

There are different types of metals.

But only the lucky see the pretty side of me.

Not sure about this. Probably it means that only the rich are able to afford pretty metals like gold and silver.

Perhaps you are:

Oil

Known since the ancient times

Oil was first used in ~4000 BC.

I'm sort of a chameleon

Chameleon has a second definition as "somebody who changes opinion or behavior according to the situation" - oil can be used in many ways.

Sometimes I am ugly

Crude oil

And sometimes worth a million

Refined petroleum

This value, however, is nothing compared

To what I can do if not handled with care

You use me to leave a mark on a tree

Pollution

Or to free life of impurity

Many medicines are based on oil, and essential oils are, well, necessary.

My family is huge, I can't say I'm lonely

Reference to the dinosaurs, perhaps? Not sure on this one. "...there are many kinds of oils like coconut oil, mustard oil, olive oil, petrol etc." - manshu

But only the lucky see the pretty side of me.

Striking oil is rare and can be costly to refine - gotta have a lot of luck.

Similar to Hackerdashi's answer, you are probably

a knife/blade.

Known since the ancient times

Knives/blades are very old tools.

I'm sort of a chameleon

Blades take on different shapes depending on their intended use.

Sometimes I am ugly

Open for interpretation, but ugly could be either the shape of the blade being unpleasant, or the use of a blade to do bad things.

And sometimes worth a million

Some blades are very valuable and expensive, such as the kukri.

This value, however, is nothing compared to what I can do if not handled with care

Knives are dangerous. Need I say more?

You use me to leave a mark on a tree

Well, that's one way to use a blade...

Or to free life of impurity

...and that's another way.

My family is huge, I can't say I'm lonely

As mentioned earlier, there are lots of different kinds of blades.

But only the lucky see the pretty side of me.

Better to be on the right end of the blade. Although, perhaps Eutherpy is being a bit dark for suggesting that using a blade for such things as "freeing life of impurity" is pretty. :P

It's people, right?

Known since the ancient times

People were known since then!

I'm sort of a chameleon

This one is kind of a reach.

Sometimes I am ugly

There are ugly people. God, are there ugly people.

And sometimes worth a million

Ophra is a people.

This value, however, is nothing compared To what I can do if not handled with care

You can raise them to be more than Ophra!

You use me to leave a mark on a tree

Family tree.

Or to free life of impurity

Babies are considered pure, I think.

My family is huge, I can't say I'm lonely

There's a lot of people. At least a million.

But only the lucky see the pretty side of me.

A lot of them are assholes.

Going more abstract:

Industry

Known since the ancient times

People have been industrializing forever.

I'm sort of a chameleon

There are many forms of industry.

Sometimes I am ugly

Gilded age, oppression of lower class by upper class.

And sometimes worth a million

Literally a million - industry makes trillions.

This value, however, is nothing compared

To what I can do if not handled with care

Once again, unregulated industry can have bad side-effects.

You use me to leave a mark on a tree

Interpreting "tree" as family tree: the Morgan family, for example. Family trees were forever placed in the state of wealth.

Or to free life of impurity

Medical industries save thousands of lives.

My family is huge, I can't say I'm lonely

Once again, industry is everywhere.

But only the lucky see the pretty side of me.

The 1% claim: only the lucky, the extremely rich get the "pretty side" of industry.

I think it's

elements, if that's not too broad.

Known since the ancient times

They've always existed, and in ancient times people believed in the four elements earth, fire, wind and water.

I'm sort of a chameleon

There are many elements

Sometimes I am ugly

They can basically take any form, so they can be ugly too.

And sometimes worth a million

Gold, platina, etc.

This value, however, is nothing compared To what I can do if not handled with care

Radioactive elements can be dangerous, see Marie Curie

You use me to leave a mark on a tree

You can make a knife or axe out of iron, for example

Or to free life of impurity

Elements can make filters or catalysts

My family is huge, I can't say I'm lonely

100+ elements

But only the lucky see the pretty side of me.

I guess maybe chemists, but I don't know why they would be lucky

The answer is

Minerals

Known since the ancient times

Have existed for billions of years

I'm sort of a chameleon

Can blend around its surroundings

Sometimes I am ugly

Not all are appealing to the eye

And sometimes worth a million

Gems are minerals

This value, however, is nothing compared

To what I can do if not handled with care

There exist a large number of fatal minerals

You use me to leave a mark on a tree

can easily be done using any cheap mineral laying around

Or to free life of impurity

My family is huge, I can't say I'm lonely

there are over five thousand kinds of minerals

But only the lucky see the pretty side of me.

not everyone has the privilege to see one in front of them

If one were to think more abstractly,

perhaps language?

My guess:

Drugs

Known since the ancient times

I’m sure some drugs were known in ancient times.

I'm sort of a chameleon

Not sure what you mean; I’ll put it down as one of your “extremely vague” lines in the riddle.

Sometimes I am ugly

Some medicines taste horrible.

And sometimes worth a million

Some drugs are expensive.

This value, however, is nothing compared

To what I can do if not handled with care

Drugs can kill if abused or if an overdose is taken.

You use me to leave a mark on a tree

Must be the other “extremely vague” line in the riddle.

Or to free life of impurity

They cure diseases.

My family is huge, I can't say I'm lonely

Neither can I.

But only the lucky see the pretty side of me.

Some drugs are obtained from plants in the wild, which can have pretty flowers.