Every old cemetery section is a decades-long experiment in how stone behaves in that exact soil and climate. Nothing in a dealer's showroom can tell you the same thing, because everything there is new.

Nineteenth-century white marble often shows a grainy surface loss that rounds off edges and erases shallow inscriptions. It is the most common reason old names have become unreadable.
The most useful research you can do before you sign a monument order costs nothing and takes about an hour. Go to the cemetery where the marker will stand, find the oldest occupied section, and walk it slowly with a notebook. Everything a dealer can show you in a showroom is new, polished, and untested; everything in that old section has already been through eighty or a hundred winters in the same soil, the same air, and the same freeze-thaw cycle your stone will face. You are not sightseeing. You are reading a long-running field trial.
Start by locating the section with the earliest dates, then check whether it resembles the plot you actually own. Look at slope, drainage, tree cover, and how close the markers sit to a road where salt and plow spray reach them. A monument on a dry rise behind a windbreak will age differently from one at the bottom of a swale that stands wet each spring. If the old section is on flat, open ground and your plot is under a maple that drops acidic litter and drips sap, note the mismatch and hunt for a few older stones under similar trees before you draw conclusions.
Sort what you see by material, because material explains most of the difference in condition. White marble from the nineteenth century usually shows sugaring, a grainy surface loss that softens edges and swallows shallow inscriptions, and the Environmental Protection Agency is the federal body responsible for tracking the airborne sulfur and nitrogen compounds that accelerate that kind of decay. Sandstone often delaminates in sheets. Slate can stay startlingly crisp. Granite, in polished and rough forms alike, tends to sit there looking much as it did, which is the single strongest argument for granite in a modern order.
Look closely at the granite that has aged well and note the color families. Dark gray and black granites hold polish and contrast for a long time; some reds and pinks fade at the polished face where sun hits hardest. Run a hand along a north-facing panel and then a south-facing one on the same stone. Where you find a marker that still reads perfectly at ninety years, write down its color, its finish, and whether the inscription faces the weather or away from it. That is a specification you can hand to a dealer.
A careful reader separates the stone's condition from the inscription's legibility, because they fail independently. Deeply incised letters cut into granite hold up almost regardless of neglect. Shallow sandblasted lettering with paint or enamel fill goes patchy within a couple of decades, and once the fill is gone the depth may be too slight to read from standing distance. Raised letters on marble lose their crispness first at the serifs. Lead-inlaid lettering, where you find it, often still reads brilliantly. Bronze plaques on granite bases oxidize to a dull brown and sometimes loosen at the pins, though the lettering itself survives.
Check the small things too: the fine outline work around a design, the shading on an etched portrait, the thin lines of a script name. Delicate work is the first casualty. If a stone you admire from 1935 has bold, deeply cut block capitals and a simple carved border, that combination is the reason you can still read it, and a dealer can reproduce it exactly.
Most monuments that look wrong after fifty years have not decayed; they have moved. Walk the row and note how many uprights lean, how many have opened a mortar joint between die and base, and how many flush markers have sunk below grade or tipped so water pools on the inscription. Then look for the ones sitting dead level and square, and see whether you can spot the concrete foundation edge at the soil line. Ask the cemetery office how deep foundations are poured in that section and what they require now. Foundation depth and drainage are what buy you a straight stone in 2090.
Take photographs with your phone, note section and row numbers, and bring the whole file to the dealer. A conversation that begins with a picture of a ninety-year-old marker and the question of how to match its stone, its cutting depth, and its footing is a far better conversation than one that begins with a catalog.
Sandstone markers tend to shed in flat sheets as moisture works between the bedding planes. Where you see it, the inscription usually goes with the sheet.