The following article is in reference to coins, however it can be adapted to suit any sort of reflective sample. Change the liquid to suit your sample type.
Because the surface of a coin has many angles and polarised light does not work well on metals, it may seem an intractable problem. Actually it’s easy: just place the coin in a suitable shallow dish and cover the coin with water – or alcohol if you are loath to use water on pristine coins.
The light now no longer is reflected by the angled metal but only by the liquid’s flat surface. Apply a light at roughly 30 degrees and the reflection exits at the same angle. Therefore your photo will be totally reflection free.
Tip: Place a coloured cloth under the dish for a uniform background.