Nothing fancy here, just a little time-saver I made for myself and decided to share.  It takes the gem value table from the Referee's Manual and adds the listed gem types for each gp value.  95% of the time you can get a gem type and value in a single d100 roll; the other 5% of the time a second d100 roll must be made to determine the exact type of gem.

GEM TYPE AND VALUE TABLE (d100)
01-03 Azurite (10 gp)
04-06 Banded agate (10 gp)
07-09 Blue quartz (10 gp)
10-12 Eye agate (10 gp)
13-15 Hematite (10 gp)
16-18 Lapis lazuli (10 gp)
19-21 Malachite (10 gp)
22-24 Moss agate (10 gp)
25-27 Obsidian (10 gp)
28-30 Rhodochrosite (10 gp)
31-33 Tiger-eye (10 gp)
34-35 Turquoise (10 gp)
36-37 Bloodstone (50 gp)
38-39 Carnelian (50 gp)
40-41 Chalcedony (50 gp)
42-43 Chrysoprase (50 gp)
44-45 Citrine (50 gp)
46-47 Jasper (50 gp)
48-49 Moonstone (50 gp)
50-51 Onyx (50 gp)
52-53 Rock crystal (50 gp)
54-55 Sardonyx (50 gp)
56-57 Smoky quartz (50 gp)
58-59 Star rose quartz (50 gp)
  60    Zircon (50 gp)
61-63 Alexandrite (100 gp)
64-65 Amber (100 gp)
66-68 Chrysoberyl (100 gp)
69-70 Coral (100 gp)
71-73 Garnet (100 gp)
74-75 Jade (100 gp)
76-78 Jet (100 gp)
79-80 Pearl (100 gp)
81-83 Spinel (100 gp)
84-85 Tourmaline (100 gp)
86-87 Aquamarine (500 gp)
88-89 Violet garnet (500 gp)
  90    Black pearl (500 gp)
91-92 Peridot (500 gp)
93-94 Deep blue spinel (500 gp)
  95    Tourmaline (500 gp)
  96    Roll again: 01-50 Emerald (1000 gp), 51-00 Fire opal (1000 gp)
  97    Roll again: 01-50 Black opal (1000 gp), 51-00 Oriental amethyst (1000 gp)
  98    Roll again: 01-50 Oriental topaz (1000 gp), 51-00 Sapphire (1000 gp)
  99    Roll again: 01-50 Star ruby (1000 gp), 51-00 Star sapphire (1000 gp)
  00    Roll again: 01-20 Diamond (5000 gp), 21-40 Jacinth (5000 gp), 41-60 Oriental emerald (5000 gp), 61-80 Ruby (5000 gp), 81-00 Black sapphire (5000 gp)

Happy rock hunting to your players!