Stalagmite Apple is the son of Marble Pie and Big Macintosh
Stalagmite's full name is Stalagmite Jonathan Apple Pie just like his mother's side of the family they have a long name, Stalagmite is a complicated boy one moment he'd be chatting with a friend or his family, and the next he's so quiet you could hear a pin drop near him, he likes to keep to himself and can come across as standoffish which is nothing he does on purpose like his dad Stalagmite is pretty strong for his size and age, he even has his own cart while it's not as big as Mac's it's big enough for him so he's able to make rounds when he needs to.
He got his cutie mark in making Stones into something really beautiful such as fruits, animals, and even other ponies he made little stone sculptors of his family and gave them to his family and sure it's a bit creepy but he meant well and it looked like he had put a lot of effort into them, due to the more feminine appearance he's sometimes mistaken for a mare, it annoys him but also finds it kinda funny. Stalagmite also loves kids sometimes he is asked to babysit his younger cousins and of course he'd always accept and Melrose, Duskstone, and Honeycrisp always love when he babysits them.
Mac and Marble adore their son to bits and they like to do things with him when they have one on one time with him with Mac they do plow racing like he and his dad used to do and with Marble it was stone cutting and sculpting, he loves doing little activities with his family as well as his cousins, though in large crowds he tends not to do so hotly and that was sort of deep-rooted fear since he was a foal he had gotten lost when he and his parents were out at the farmers market he was about 4 at the time and it was terrifying for him and feared he'd never find his parents but luckily he ran into Fluttershy and with her help she managed to get him back to Mac and Marble who was relieved he was alright, ever since he'd get thrown into a panic attack when he's in large crowds.
Now as for how his parents got together is kind of a heartbreaking story for both of them Marble seeing Mac kissing his mare friend at the time Sugar Belle when both their family's were celebrating Hearth's Warming and after Marble had spent years working up the courage to confess her feelings to him only to have that one and only chance to get kicked out from under hooves leaving Marble heartbroken and devastated but just hoped he was happy, but she had closed herself off from everyone and her work suffered due to it which led to some intense fights with her parents until she finally snapped and left the rock farm to live with Pinkie and to get away from her family which Pinkie felt angry and sad for Marble.
For Mac his was a bit different but did end up leaving him a heartbroken stallion, about two or three years after their wedding Mac had been noticing Sugar Belle going out at a specific time and coming home late or not until the next morning, Mac found it kinda weird but didn't want to jump to conclusions since he didn't want to repeat what happened the last time it happened but the more it was going, the more worried he became and so one day he was out to pick up something for AJ he noticed Sugar Belle with a stallion at first he didn't pay much mind to it but when they started to kiss that's when the pieces began to put themselves and when she saw Mac was there she tried to explain but at the moment he just didn't want to hear it.
Marble had found out what Sugar Belle did to Mac to say she was furious was an understatement and she was there to comfort him while Sugar Belle was ostracized for what she did by not just Mac's family but all of ponyville, even the stallion was disgusted by what she did and dumped her and so she left ponyville and went somewhere else, but before she did leave they got a divorce and Mac won the case soon Sugar Belle was no longer in ponyville in fact no one knew where she went but at that moment the damage had already been done and throughout it all Marble didn't leave his side and Mac really appreciated it and soon feelings for one another began to bloom but both were a bit afraid of it turning out like Sugar Belle all over again and so they when they both felt they were absolutely sure about it as well as taking time to make sure they had officially started dating and that's when Marble finally told Mac how she really felt about him and he did as well.
Soon they were engaged and then married Marble's wish had come true she was with her Prince Charming and at that time Marble cut off from her family expect for Maud, if they could reach the four-year mark in there marriage then that would be a good sign especially since Mac's didn't last for more than two or three and luckily enough they did reach the four-year mark, then Marble announced she was pregnant Mac was over the moon as well as his family. Months of normal symptoms of being pregnant, she gave birth to their firstborn son which they named Stalagmite Apple.