Refractive index of gems

Hello,
Why are there so many different values for the refractive index of a specific gemstone on websites? Which website gives the true refractive index range? If I look up the refractive index range of a gemstone on three different websites they all give different highs and lows for the range.

Welcome Richard!

This article hints at the problem.

I came across it while taking the minicourse on Gemological Tools:

Cheers!

-Troy

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