Approximately 12mm. If made into a bracelet, the inner diameter would be about 14.5cm. If you'd like it made into a bracelet, I'll assemble it for free, so please feel free to ask. :) You can also combine it with other stones in the store. Perfect for handmade crafts! It's 50 yen off for every two items, so please take advantage of it! To prevent trouble: There may be natural cracks, scratches, dents, and inclusions that are characteristic of natural stones. ◡̈⃝︎⋆︎* There will inevitably be an error of about plus or minus 0.5mm, so I've written approximately ○mm. There's nothing as perfect as artificial glass; please understand this before purchasing. ◡̈⃝︎⋆︎*