A coin, as its name implies, is an 8mm diameter circle that is only a few mm thick. It is just thick enough to drill a hole from one side to the other along its diameter.

The 8mm round is a sphere with a volume that contains more gemstone material than the coin. In a necklace photo, the round may not look much larger than the coin.

A rondelle is basically a coin on end with a hole drilled through the middle. On a string of beads, you will see its edge. Some rondelles are thicker in the center than at the edge. Others are almost as thick as a round.

The 8mm flat square is the same thickness as a coin, but its corners make it a little larger.

If a square is drilled from corner to corner, it may be called a diamond.

The 8mm cube bead has the greatest volume of any of these beads I have described. The cube can be drilled from side to side or from corner to corner.

The corner drilled cube takes up more space in a strand and will extend out farther from the center.

As you can see, even though all of these beads are labeled 8mm, the shape makes a big difference in their actual sizes.

Shapes whose lengths and widths are not the same need both measurements to visualize accurately. Use 8mm as length and 6mm as width. These 8 mm length beads will occupy the same space in the strand as the other 8mm beads but will extend out less from the wire.

The oval will be larger than the flat oval.

flat oval bead. 

The rectangle will be larger than the flat rectangle. Because of their corners, the rectangles are larger than the ovals. Long rectangles may be called square tubes.

flat rectangle 

A diamond that is 8mm long and 6mm wide may be measured from point to point. It would be smaller than the flat rectangle.

A puff oval, rectangle, square or coin are flat beads that puff out to a larger thickness. Puff rectangles may also be called cushions or pillows. The corners are sometimes rounded.

puff rectangle, cushion 

A barrel bead, as its name implies, look like a barrel tipped on its side. It has more volume than the 8mm long oval.

Tube beads are accurately named. Their length is usually much longer than their diameter. The tube can be round or square.

Rice bead is a name given to a long oval that tapers to small ends.

Although in jewelry photos the flat beads may look nearly the same size as their larger volume counterparts, when you actually see them, the third dimension will change their appearance.

There are three common bead shapes that are irregular.

Chips are similar to rondelles. They are thin and extend out from the bead wire. As the name implies they can be the chips that are left when a gemstone shape is cut.

Pebbles are irregular in length, width, and thickness, and are difficult to measure. Usually they are sorted into a size range, such as 7-9mm. Their shapes resemble the rock pebbles that you might find on a beach.

Nuggets are similar to pebbles, but have a more rectangular and regular shape. Both pebbles and nuggets will probably look smaller in a photo.