When buying gems at a jeweler's, one must be aware that there is a vast array of different colors and cuts. The chart below is a very simplified list of gems, colors, percent chance to find one for sale, and base value. [Please note this is kind of based on real-world rarity, but not intended to reflect true rarity or value. This is particularly true for pearls, which are mostly cultured, farmed, colored or bleached in the real world, but not in this game.]
To see if a particular gem is available, roll percentile dice, and if the number rolled is less than or equal to the number on the chart, one such gem is available. To check to see if a second gem is available, subtract 5% from the number and roll again.
If the gem needed is of a higher base value, reduce the chance of success this way: for a gem up to twice the base value, reduce chance of finding one by 25% of the base; for up to 5 times the base value, reduce the chance by 50% of the base; for up to 10 times the base value, reduce the chance by 75% of the base, for gems more valuable than that, reduce the chance by 90%.
Note this is per store, per month. Some large cities may have several jewelers, some will have one or none.