Legally, most diamond treatments have to be disclosed. This makes it very important that you know you are dealing with a reputable company, one that would not risk falsifying information about their diamonds just to get the sale. So long as a treatment is properly disclosed, there is nothing inherently wrong with enhanced diamonds.

It's just that their value is less than that of a diamond that is naturally less flawed. As treated diamonds are cheaper, they can be nice for the more budget-minded shopper. It's a basic fact of gemstones that higher quality stones are rarer and more expensive. A lower quality stone that has been made to appear like a better one is not deserving of the higher price.

Laser drilling is a treatment to remove inclusions. In a lot of ways, this is not so bad a thing, except that it makes the diamond just a touch more fragile. Nothing too significant, but this is a hole in the diamond, after all. You will want to be sure that the price is lowered to reflect this.

Fracture filling is another treatment to improve the appearance of a diamond. The treatment is pretty good, except that under high heat (such as when a diamond is being set), the fill material can melt away.

A diamond can also have a very temporary treatment that changes its color. A little spot of blue or purple one the culet can make a diamond appear whiter than it really is, if it is really just a touch yellow.

As a rule, really good diamonds won't be treated at all. For the occasional small flaw there is really no need. The value of the diamond is high enough without the treatments.

Your best reassurance that you know all of the treatments applied to a diamond is to get a certificate from one of the three major labs: GIA, AGS or EGL. Any of these labs will note treatments the diamond has gone through on their certification of the diamond. If you're investing in a high quality diamond, this is one of the best ways to ensure that you are getting your money's worth.