Wow, blast from the past!
So here's an update, because you all seem interested: in the three years since I posted this, yes, her father's soul diamond became her lightsaber crystal. She entered a temple on the ruined Jedha where she faced one of her trials, and in the process invoked the lessons her father taught her and the ideals her mother left to her. The end result was her father's soul diamond fusing with her mother's (which she had since retrieved and was carrying on the same necklace) to become a Kyber crystal. These are its stats:
QuoteBase Modifiers: Installing this crystal changes a lightsaber's base damage to 6 and critical rating to 3, and the lightsaber gains the Breach 1 and Sunder weapon qualities. When making a Force power check when this lightsaber is activated and in a character's possession, said character may spend Force points to recover 2 strain. If the crystal is ever removed, the lightsaber loses these qualities and abilities, and reverts to its previous base damage and critical rating.
Modification Options: 1 Decrease the weapon's critical rating by one to a minimum of 1 Mod, 2 Damage +1 Mods.
Hard Points Required: 2.
I kept it a little less formidable than an Ilum crystal for a couple of reasons. First, she already found her mother's lightsaber, which was left in the care of a family friend while the PC was growing up, and that had an Ilum crystal already. Second, I wanted to give it a unique (well, "unique;" pretty sure I copied the strain recovery from one book or another) power that could be a good buff. Plus, I knew she wanted to build the hilt from scratch, which meant the chance for plenty of additional bonuses.
Thematically, it worked great. A strong undercurrent of this game has been family: the different strengths of the family you're born with vs. the family you choose, how our families define us, and how our choices define our families. I said that recovering strain with the crystal could be narrated as a soothing sensation of familiarity, a whispered word from her mother or father, a smell she associated with her childhood, something like that. She also chose to make the soul crystal lightsaber her primary weapon, reforging her mother's old lightsaber into an off-hand shoto.
I've put the crystal's connection to her family to work several times, with various visions and words. (At one point, I made the player cry with a well-placed, whispered "We're so proud of you," which I guess is a good thing?) Recently, the crystal has been responsible for a series of visions from her mother's past, which is shedding light on the unfolding mystery of the campaign. I'm also planning for the endgame (which should happen at some point this summer) in which the PC's confrontation with her former (now fallen) mentor will see her enemy try to corrupt the crystal and so manipulate the PCs.
So for everyone who has been encouraging this, thank you. To anyone who has been considering adding an element of this to their campaign: do it!