Two pieces, typically mitered, cut with a wet saw (most often for me), or sometimes a dry diamond blade on a grinder or circular saw (least often for me).

Since the curb is membraned and the curb is pitched to drain into the shower, the marble seam would usually be grouted, but you could use color-matched caulk if you desire.

What does 'membraned' mean? I read that an 1/8 in. pitch is common. Does that mean the slab is flat and cemented in entirely at that pitch, or do marble thresholds for showers come with the pitch already in them with a flat bottom? Wouldn't the glass sitting on top of it need to meet the slab at a flat place and not a place that is pitched? Thanks again.

I'm not sure how your shower is being built, but in a "typical" shower the shower pan membrane should extend from the clamping drain, up the sloped base of the shower floor to the walls and then up the wall. Where there is a door instead of a wall the membrane will go up the sloped base to the shower curb and up and over the curb.

This is an excellent schematic...thanks. Yes, I understand what you mean by the membrane now... it will extend over the entirety of the curb. I guess I had a few questions in one. I was wondering more about the slabs themselves... if they come already angled on top (so as to provide for pitch towards shower), or if they are flat on top and bottom and you cement them in at an angle.

If you do use a standard flat piece curb, you set the pitch when installing them. It's one of the few times when it's okay to install something "crooked". ; )

A fabrication shop can make them however you specify, but you'll pay extra. Sometimes you can find marble saddles or curb pieces that do have an extended bevel on each side. Depends on what the local places carry as "standard".

Setting glass on a pitched curb top is not a problem.

If you do plan on using ready made marble pieces, look of one with an extended bevel. A standard bevel may only be a 1/4" or 3/8" 45-degree chamfer on the edge of the stone.

A "hollywood bevel" is an extended bevel, so it's a longer area of slope.

Then there are "double hollywood bevels", where each long edge has a wide bevel.

A picture may be worth more words, lemme see what I can find...here's a hollywood bevel

Hollywood bevel:
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While most thresholds are 3/4" thick "door" thresholds, you could use those, or you can get the same material and style in thicker material if you want a thicker curb.

I think I would probably like the look of a flat-top slab with the entirety sloped rather than get into beveling. Since you've been so helpful, can I try my luck with you one more time to ask if you know of a fairly common marble that is in the light beige category rather than white? Or... maybe a white slab for the curb, around a shower and floor all done in beiges would be a nice accent, and compliment the white toilet and vanity top. What do you think? Anybody else feel free to chime in too! It's a pajama jammy-jam!