Approximately 10mm. It shimmers and sparkles every time light hits it♡ The needles are also plentiful, making it an exquisite quality. Perfect for your handmade projects! Get 50 yen off for every 2 items purchased, so please take advantage of it♡ To prevent any issues: Due to the nature of natural stones, there may be natural cracks, scratches, dents, and inclusions ◡̈⃝︎⋆︎* There will inevitably be an error of approximately plus or minus 0.5mm, so the size is listed as approximately ○mm♪ It is not perfect like artificial glass; please understand this before purchasing ◡̈⃝︎⋆︎*