Threads for bead crochet have two required qualities: strength, and stability. There can be no stretching and no breaking. Certain kinds of cotton and nylon have these essential qualities.

I do love the feel of cotton for bead crochet. Perle cotton and Mercerized cottons are strong and smooth, have little stretch, and work beautifully for crocheting with beads. Perle cotton (original correct spelling: perle coton, French) is available in a side range of sizes. For size 11 seed beads, size 12 perle cotton is a good choice for tubular, tapestry and Euro bead crochet. If you are bold enough to try size 15 seed beads, you can use 20/2 mercerized cotton.

In most retail stores you're likely to find only white or ecru in most of the weights; visit my sourcing page for a list of retailers that offer a good color range on line. The put-up is usually in a ball, quantities vary by the gram weight. On the label of the ball you will generally find information about how many yards/meters are in the ball and with that information you can determine how many you will need. A fifty-gram ball of size 12 perle cotton will create up to ten bead crochet bracelets in tubular bead crochet, perhaps four in Euro bead crochet. Tapestry bead crochet projects will generally give the needed amount in the directions.

Nylon thread that is generally used for upholstery is also a good choice; it's a forever thread because it's really plastic, made into fibers that are then spun together to form thread. The best weight for light bead crochet is bonded nylon size 69; it's fine enough to go through a threading needle and will fit through beads size 11o or larger. Bonded nylon size 33 can be used for 15os; I like it for earrings because it is very lightweight and extremely strong. Most nylon threads come on cones as their original purpose is for large sewing projects, often outdoor projects like sails, boat cushions and canopies that require massive amounts of thread. You can wind off what you need and put it on a winding card.