Almost any decent quality enhancer will do the job. Home Depot carries some good products, just don't buy the cheapest ones. Most enhancers have a choice of gloss or matte finish. Since it's indoors you might look for one that is low in VOC's to cut down the odor when you put it on. Since the degree of color enhancement can vary greatly between different types and colors of stone (and even different brands of enhancer), I would suggest that you buy a small container or two and test them on a scrap piece (or if you don't have any scrap, on an inconspicuous spot) to see if you like the effect. Also, a quick way to get an idea of what it will look like is to moisten a section of the stone with water - typically the moistened stone is what it will look like after the enhancer is applied.

Thanks- I have looked at HD and the enhancer they have is a miracle brand which I had seen online but it does not say if it is matte or not. Since it is 40 some dollars a quart I may try to contact the manufacturer to find out its sheen before I buy it. I really hope that the enhancers will give the rock the same richness and depth that they have when wet- it is such a huge difference!
Do you have any idea on how long I should let the grout cure before I use anything on it?

Then go to Lowe's or a local stone/tile store and get an enhancer that lists the sheen on it. I know they are out there as I have used several different brands that do list the sheen. The result should be close to (but not quite) what it looks like when wet. It will usually say on the grout container how long the recommended cure period is. The minimum cure period I've seen is three days. I've also seen some that recommend four weeks! If you don't have the grout packaging, I think you're okay to seal it after a week. Also, depending on the porosity of your stone, a second coat of enhancer will make a difference (if the first coat absorbs quickly, then you should definitely apply a second coat).

Do you think that there is much chance that I could screw up my fireplace by putting enhancer on it? When I called the stone center where I bought the rock they said that it may not absorb evenly but it seems that with enough coats it should be ok. And the guys who installed the rock have not heard of using anything but linseed oil which I am not even going to attempt. It does not seem that crazy to use a color enhancer but both of the people I spoke with advised against it.

What kind of stone is it? The only complaints I've heard from people using enhancer is that it didn't do much - which is what happens if the stone is very dense like some granites. It's true that some stone might not absorb evenly but that's because they are made up of a combination of minerals. Therefore, the parts that might absorb unevenly are a different shade or color to begin with so it just enhances the contrast but doesn't look bad. On the other hand if the people who sold you the stone are concerned, that's all the more reason to test it on a couple sample pieces/scraps before applying to your fireplace.