Materials
Bronze
Ancient singing bowls were traditionally made out of “Panchaloga” which is an alloy made out of five different metals that held spiritual significance. These traditionally were gold, silver, brass, iron and copper. In modern times the alloy mix differs, and most modern singing bowls omit gold and and silver due to expense. The mix of metals is considered bronze, and this is what the majority of modern singing bowls consist of. These are also what is commonly referred to as “Tibetan Singing Bowls”. This kind of bowl not only sounds great, but is keeping with a great tradition of singing bowls that go back hundreds and hundreds of years.
Crystal
Crystal bowls are usually made out of quartz crystal. They have a crisper tone and are a more modern type of singing bowl. This is probably the second most popular type of material behind bronze, and is gaining popularity quickly due to the tight, clear sound. Another great feature of crystal bowls is they can be combined with many other metals, gems and minerals. For instance, there are bowls made from quartz and ruby that are simply beautiful. This allows for many different tones and appearances.
Other materials
Though bronze and crystal are the most common type of bowls, singing bowls have been made from all sorts of different metals and materials, each with their own distinct sound. You can even buy bowls made out of expensive materials such as gold or platinum!
Sizes
Singing bowls come in all sorts of sizes. The rule of thumb is - the smaller the bowl the higher pitched the sound. If you’re a first time buyer, you may want to try several sizes to see what is most pleasing to you (or listen to sound clips if buying online). Many people own several bowls for a wide varity of sounds. Also, several stores sell sets that contain several sizes in a uniform apparence.
Pitch
When choosing a tone from a singing bowl, you have to ask yourself what purpose you’ll be using the bowl for. If only meditating with a single bowl, then the pitch may not be important. But if using the bowl for music, or playing with other bowls, then you will want to pay attention to the pitch. Many bowls aren’t exactly on pitch - for instance, a bowl which sounds the note “C” may be slightly above or below a perfect “C” played on a tuned instrument such as a piano. You may want to look for a shop that sells perfectly tuned bowls. This way you can play with other bowls and other other instruments. I highly reccomend this, as even just playing it with another bowl in harmony is moving.
A note on harmony: Not to delve too much into music theory, but if you are looking to select several bowls to play together it is important they are in the right relation to each other on the music scale. Many singing bowl users like to buy 3 bowls which would form a chord. For instance, a major chord is formed from the I, the iii and V of a scale. A popular chord is C Major, in which you would buy bowls in C, E and G.
Hopefully this clears up the differences between bowls. I wish you the best in your foray into the wonderful world of singing bowls!