Note: every time Alice transforms the necklace disappears, just having the gems of the creatures she transformed attached to her chest (or whatever part that fits for that form).
Alice returned to normal touching the red gem again, both of you were naker laying on the bed, you looked at her necklace. "What if you try more than one gem at the same time?" You curiously asked noticing that you only tried one gem at time.
"That's interesting, let's do it!" Alice touched both the sapphire and the diamond, quickly turning into the creature she expected. Her legs multiplied resulting in 8 limbs that got longer, purple and sticky becoming tentacles in her lower body, her arms grew black hair and thinner transforming into bat wings, her ears became pointy and her mouth grew fangs like a bat. "This is truly awesome, I love being a freak!" She excitedly said.
"Wow, you look amazing" you saw your sexy monster girlfriend, noticing she had the sapphire and diamond incrusted in her chest. "What more can you do?".
Alice touched both gems again and turned to her naked human form, "I don't know, but I have another idea" she hold her finger on the emerald instead of just touching it. Her body grew another pair of legs, instantly rearranging in her body, one pair grew yellow and black striped hair, while the other pair grew a light grey hair, each pair grew different forms of claws. Her form was essentially a centaur, but her frontal legs were the ones of a tiger while the hind legs were wolf like, in addition her arms grew white furs and grew a pair of goat horn in the top of her head, just having the emerald attached to her chest, showing a "III" symbol, the number three in Roman.
"I can even combine multiple creatures of the same element, this is more than amazing! you definitely gave me the best gift ever" she said, approaching you with her tauric body.
You couldn't hide your erection, which Alice smiled just looking at it.
"I want to try one last thing" Alice regressed to her human form again, Alice touched the ruby, curious of what did that represent.
Alice grew extremely bigger, with her skin turning red, her muscles bulking and her face turning more macabre with long fangs and the ruby in her forehead, she turned into a female Oni. The ruby actually represented the monsters (demon, oni, ghost, dullahan, manticore, etc) instead of the rest of the gems that represented animals of different elements.
"Do you like what you see?" Alice posed in her big and monstrous form.
You sweated, looking how your girlfriend started to gain control over her new powers, things could get really interesting.