Oops, I almost forgot to mention - the stone has a deep scratch in it on the corner. Anyone have any ideas on how to make it less noticeable or fill it in?

Any stone of that size is heavy. Its too haed for me to identify at this distance. One needs to see and feel the stone and possibly make a hardness test to get an idea of what it may be. It does not appear to be granite; The grain and constituents are wrong. Granite may have grain size from microscopic to 1/2 inch in size. There should be a percentage of mica showing. Your top has some rather large "rocks" in the mix although these may be quartzy materials.

Your top may be a "manufactured" stone. It would be made of a mixture of small stones, stone chips, and stone dust cemented together, cut, and polished. The cement may be regular cement (same as in concrete) to epoxy resins. Fiberous fillers may be added to strengthen the mix. Some Italian manufactured stone does a very good job of simulating natural stone.

To get an idea of what you may have, visit a builders supply house and look at various stone tops and learn about compositions. In my area, a few stone quarries maintain indoor showrooms of polished stone from their quarry and display ideas of how stone can be used in your home.

Would manufactured stone have been used so early? My best guess is that this piece is from 1870-1890, and the top is definitely original (it was owned by only one family and was never moved or altered from the house it was in when it was first purchased.

If you run your fingernail along the top of it, you can feel differences in the surfaces of the stone. The gray area in the middle of the stone and the pinkish areas feel different to me, for example.

Going to a showroom is a good idea, I may give that a try. Thanks!

Likely that stone is not original to the piece. All the furniture I have seen with stone tops of that era were marble. I would replace the top. The piece appears to have been refinished, so it would be no detriment to the value.
You saying that was in the same house for 130 or 140 years?
Amazing!
Linda C

It's worth getting in a stone repair specialist - they do a lot of countertop work. A swirled bit of epoxy that blends with the surrounding stone for the scratch and one of their epoxies for the chipped-off corner.

Marble VS Granite is hard to tell, but marble was easier to work, therefore much more common. And testing would damage the top. Just admire it.

LindaC - If it's been well cared for, that could be the original finish.

I disagree about this top not being original to the piece because I had an Eastlake table with a top made of this same stone, which my great aunt referred to as rose chip marble. (This name may be completely inaccurate) but the top was clearly original.

As is the case with many more delicate pieces of Eastlake, my table was very top heavy and wobbly because of the weight of the top on the relatively spindly legs. One of the legs broke one time too many and I kept the top but disposed of the table.

lindac - The flash on my camera is rather strong and makes the sideboard look more orange than it really is. I spent hours bent over this thing cleaning out the dust, and I have to say that I am 99% sure the finish is original. In person it's a darker, less reddish color if that helps.

The sideboard's origins are interesting. I bought the sideboard from a charity, and its director gave me the background on it. The house the sideboard came from had been in the same family since it was built in the 1800s but had been vacant for many years, and the last surviving family member/owner died recently. After that death the house was slated for demolition, and the nicer pieces of furniture that were found inside the house were donated to the charity for resale in their eBay store.

There is a mark on the underside of the stone, and I wish I had taken a photo of it before we placed it on top of the sideboard. It's a name or initials chiseled into the stone in an old-fashioned cursive script.

Run it by the kitchen forum. If it's something that's still being quarried, there's a good chance someone there will know it.

There is a family of rock known as "metaconglomerate", AKA a metamorphosed conglomerate stone. A sedimentary conglomerate is like a sandstone with pebbles; when it is metamorphosed by heat and pressure, there can be elongation of the larger particles, and re-crystallization of all of the components. I think your stone is from this family; do a google images search on metaconglomerates.
Casey

I also think it's real stone and a conglomerate. The cabinet looks like mahogany also. One of the better examples of Eastlake. I don't like a lot of Eastlake but some I do. I've worked on some nice pieces of Eastlake, replacing missing parts, and they were mahogany, one of several woods common to Eastlake furniture.

I've seen higher quality pieces (like yours appears to be) with similar tops. Casey is spot on as to the stone IMO. The finish appears to be original as well. Lovely piece...go to a stone dealer and have the top repaired so as not to destroy the value.