Recently, diamond vein meta was changed.

Now, per Wingzero, the changes consist of:

• Diamond veins only being 3 Y levels tall now.
• Diamonds spawn in narrow y-level discs instead of rooms.
• Diamond veins have a certain amount of "waviness"
• You will never find diamonds more than 3 blocks directly above or below another.

Now, this is all well and good, but there is some nuance to it. Especially with diamonds being found on only 3 Y levels as some suspect based on a short reading of the post.

The following is what I've been able to deduce and how I managed to make 50 diamond blocks today

• Diamond veins are oval shaped generally speaking, close knit, and about 7-8 blocks top to bottom usually. Here are some pictures of my recent expedition showing this.

• Diamonds in said veins typically can be found, per wings post, in a 5-7 y level range of each other, but most will be found pretty close knit in y-range.

• Diamond veins seem to have "straggler" ores outside of said oval. Unsure if these are different veins, but I dug pretty far out from them and I can assume that they are simple the exterior of the "ring"

• Most diamond veins are found between y-4 and 7-8 in my experience.

• Veins yield usually between 1 to 2 runs worth of diamond ore if mined properly it seems. Either that, or I'm not mining them properly.

How To Properly Mine Them

• Take a rail to a less populated region of the map, even though diamond veins have reset and can spawn under cities now, a lot of cities have underground infrastructure you don't want to mine into, or empty space that veins don't spawn in.

• Go down to Y-4 and mine a 6 high tunnel until you sense a diamond

• Dig, generally speaking, a "room" focusing on the areas that fit the shape of an over (the sides of the room, the exterior walls moving in almost a "circular" direction.

• Make sure to hit the roof and floor of it too on your way out, sometimes diamonds spawn in lower or higher Y levels, but not in mass.

• Mark all your diamonds on your minimap and you'll start to get an idea of the shape of the vein and where to concentrate your efforts on in said vein.

UPDATE

I did some field research on a new vein and found what I think to be is a "full vein" I.E. the examples I posted above may only show part of the whole, but still shows the general outline of what you're looking for.

https://imgur.com/a/si6DRDY