Just noticed your material screen - for your test it probably is too late now but the diamond refraction value is set to low. Also, do you need to adjust the refraction value as well as the refraction value?

Gemstones (Index Of Refraction):

Agate 1.544 - 1.553
Alexandrite 1.746 - 1.755
Almandine 1.75 - 1.83
Amber 1.539 - 1.545
Amethyst 1.532 - 1.554
Ammolite 1.52 -1.68
Andalusite 1.629 - 1.650
Apatite 1.632 - 1.42
Aquamarine 1.567-1.590
Axenite 1.674 - 1.704
Beryl 1.57 - 1.60
Beryl, Red 1.570 - 1.598
Chalcedony 1.544 - 1.553
Chrome Tourmaline, 1.61 - 1.64
Citrine 1.532 - 1.554
Clinohumite 1.625 - 1.675
Coral 1.486 - 1.658
Crystal 2.000
Crysoberyl, Catseye 1.746 - 1.755
Danburite 1.627 - 1.641
Diamond 2.417
Emerald 1.560 - 1.605
Emerald Catseye 1.560 - 1.605
Flourite 1.434

Garnet, Grossular 1.72 - 1.80
Garnet, Andradite 1.88 - 1.94
Garnet, Demantiod 1.880 - 1.9
Garnet, Mandarin 1.790 - 1.8
Garnet, Pyrope 1.73 - 1.76
Garnet, Rhodolite 1.740 - 1.770
Garnet, Tsavorite 1.739 - 1.744
Garnet, Uvarovite 1.74 - 1.87
Hauyn 1.490 - 1.505
Iolite 1.522 - 1.578
Jade, Jadeite 1.64 - 1.667
Jade, Nephrite 1.600 - 1.641
Jet 1.660
Kunzite 1.660 - 1.676
Labradorite 1.560 - 1.572
Lapis Lazuli 1.50 - 1.55
Moonstone 1.518 - 1.526
Morganite 1.585 - 1.594
Obsidian 1.50
Opal, Black 1.440 - 1.460
Opal, Fire 1.430 - 1.460
Opal, White 1.440 - 1.460
Oregon Sunstone 1.560 - 1.572
Padparadja 1.760 - 1.773

Pearl 1.53 - 1.69
Peridot 1.635 - 1.690
Quartz 1.544 - 1.553
Ruby 1.757 - 1.779
Sapphire 1.757 - 1.779
Sapphire, Star 1.760 - 1.773
Spessarite 1.79 - 1.81
Spinel 1.712 - 1.717
Spinel, Blue 1.712 - 1.747
Spinel, Red 1.708 - 1.735
Star Ruby 1.76 - 1.773
Tanzanite 1.690-1.7
Tanzanite 1.692 - 1.700
Topaz 1.607 - 1.627
Topaz, Imperial 1.605-1.640
Tourmaline 1.603 - 1.655
Tourmaline, Blue 1.61 - 1.64
Tourmaline, Catseye 1.61 - 1.64
Tourmaline, Green 1.61 - 1.64
Tourmaline, Paraiba 1.61 - 1.65
Tourmaline, Red 1.61 - 1.64
Zircon 1.777 - 1.987
Zirconia, Cubic 2.173 - 2.21