What Is This Discoloring Caused By?

Actually that looks pretty nice.

Toning does represent damage. I do not recommend that it be removed though because the only way to do it is to remove the "toned" metal, which means removing part of the coin's surface

You can always try "MS70 coin cleaner" if you want to try to remove it. But you always take a chance on ruining the coin when you mess with it.

It is an attractive example, so it should be left alone.

I appreciate the feedback and sound advice. I'm still very early in the ancient coin collecting hobby and still learning all the jargon. Personally, I would hesitate to buy a cleaned coin and would likely never alter a coin to get a more "desirable" finish. I guess my question was more of a backhanded way of learning what is good and what is bad "patina" or "toning."

The term "toning" kinda sounds like a euphemism a coin dealer would use to justify a particular finish in order to sell a coin. Maybe I'm just too cynical.

My advice would be to buy auction catalogs that focus on ancients, which can often be had for cheap in bulk. This can give you a good sampling of many coins: the range of what they look like, the prices, and what the cataloguers say about the coins (good or bad). Alternatively, you could set up searches on a site like NumisBids and save stuff you find of interest for comparison over time.

Silver tarnishes over time, especially over 1500+ years. Unless an ancient is nearly black in color, it was cleaned at some point -- perhaps by a museum involved with a find. Some people desire the blast white look, some desire dark toning, and some pursue colorful toning; good & bad are often in the eye of the beholder. However, toning can act as a protective shield of sorts -- something recently cleaned may turn ugly after you buy it, and there's no way to predict whether that will happen.

At some point, the word "cleaning" became synonymous with "dipping in acid." True cleaning -- conservation & preservation activities -- is an integral part of ownership. The extent to which it's needed depends on the composition & how fresh the coin's surfaces are. You shouldn't need to do much with ancients compared to medievals or more modern copper alloys, though.

There are a lot of euphemisms utilized by dealers to sell coins. You're not cynical at all to recognize that many people are out there with the sole purpose of exchanging their product for your money. Learning the language is another reason I'm a fan of auction catalogs.