Now that we can make jewelry (if we have Crystal Creations), it's possible to give Sims their birthstones, if you have Seasons.
The traditional correspondence is this:
January - garnet
February - amethyst
March - aquamarine
April - diamond
May - emerald
June - pearl
July - ruby
August - peridot
September - sapphire
October - opal
November - topaz
December - turquoise
We don't have garnet, aquamarine, pearl or peridot among our crystals, so I'm substituting rose quartz for garnet, quartz for aquamarine, moonstone for pearl, and plumbite for peridot. These are all base game or CrC.
The next question is: when is your Sim's birthday? The fast answer is to look on the calendar. That won't be the actual day your Sim was born, but if your Sim started as a young adult, it's as good a date as any. Of course if your Sim is born in game, just make a note of that date. As for correspondences to months, they're as follows:
Jan - Winter 1- 9, 1-4 or 1-2, depending on whether your seasons are 28, 14, or 7 days in length - rose quartz
Feb - Winter 10 -18, 5-9, or 3-4 - amethyst
March - Winter 19 - 28, 10 - 14, or 5-7 - quartz
April - Spring 1-9,1-4, or 1-2 - diamond
May - Spring 10-18, 5-9, or 3-4 - emerald
June - Spring 19-28, 10-14, or 5-7 - moonstone
July - Summer - 1-9, 1-4, or 1-2 - ruby
August - Summer - 10-18, 5-9, or 3-4 - plumbite
September - Summer - 19-28, 10-14, or 5-7 - sapphire
October - Autumn 1-9, 1-4, or 1-2 - fire opal
November - Autumn 10-18, 5-9, or 3-4 - orange topaz
December - Autumn 19-28, 10-14, or 5-7 - turquoise
Also, jet was used for mourning jewelry in the Victorian era, so if you're playing a Black Widow challenge, your Sim should definitely be wearing jet.
Besides the places they mentioned where you could find crystals (the parks and neighborhoods in Oasis Springs and Willow Creek, on the computer, and asking colleagues in the Scientist career if you have Get to Work), you can also find them on a vendor's table at the Flea Market if you have City Living, and there are at least 30 dig sites around the Science building if you have Get to Work.
