These gems are said to be the seed of all creation. They are the embodiment of life and chaos. The gem itself is the size of humans thumbnail, and they continually change colors and shades, pulsing with brightness. There is no telling what creature could be unleashed from these gems.
Once a gem is thrown and the command word uttered, it will spring into life as a randomized creature. This is usually a last-ditch effort, as there is an equal chance you could summon a giant fly, a giant squid, or a colossal red-dragon. Once summoned, the creature persists for 1 minute, that's 10 rounds, in which it assists the creature which summoned it to the best of it's ability.
These can never be reproduced by mortal means.
Strong Conjuration; CL 10 (Can never be created by mortal); Craft Wonderous Item, Godhood, Summon Monster 9, Wish, Price 5000gp
To determine what kind of creature springs forth from the gem take the following steps
Roll 1d20 to determine the CR, if 20, flip a coin to see if it is above 20 and then roll 1d10 and add that to 20.
Once CR is determined roll 1d10 for source.
|1||Monster Manual 1|
|2||Monster Manual 2|
|3||Monster Manual 3|
|4||Monster Manual 4|
|5||Monster Manual 5|
|6||Monsters of Faerun|
|7||Bestiary of Krynn|
Once the source is determined you have narrowed down a pool of creatures within that source that fit that cr. Then simply pick the appropriate die to determine the lucky creature from that pool. I.e. if there are 6 cr 10 creatures in Monster Manual 2, roll a d6 and the creature that comes up is your winner.