Homebrew Cursed Gem Species Details

Ability Score Increase. Gems vary widely in their innate capabilities. Your ability score changes vary by Gem type.
Age. Gems are typically several millennia old, though they may be only a few hundred years old when they explore the world.
Alignment. Cursed Gem, as former Demons may have any alignment, but typically have evil alignments due to them being captured in the first place.
Size. Gems come in a wide variety of builds. Your default height, weight, and size vary by material type.
Speed. Your base walking speed varies by material type.
Gem Physiology. Thanks to your lack of a physical form, you are is immune to poison and the effects of aging. You are considered a construct and do not need to eat, breathe, or sleep. You only need to rest for 4 hours to gain the full benefits of a long rest. If any part of your body enters an antimagic field, it scatters and disappears; if your gemstone enters the field, your entire body “poofs”.
Gem Poofing. When your HP reaches 0, you are "poofed" and retreat back into your gemstone to recover. All equipment and armor you were carrying are dropped and smoke covers the area immediately around you. You are considered unconscious and have an AC of 17 & hit points equal to half of your max HP. Damage remaining after the final blow carries over to this HP. You recover 1 HP every 5 minutes, and when the HP of the gemstone reaches your max HP you may reform. This increases to 1 HP every 4 minutes at 5th level and 1 HP every 3 minutes at 10th level. If your gemstone falls to 0 HP and stabilizes, or you are revived after being shattered, you begin recovering HP after 1 hour.
The Gemstone. Any Gem's gemstone functions as the core of the Gem's being and the focus for their magical traits. The gemstone is large enough to fill an average humanoid’s palm and is of a consistent size & shape. Your gemstone cannot be shattered while your material form is active, but it can crack. Gemstones can be carried in another's Bag of Holding when you have poofed. At your DM's discretion, the Gem’s gemstone may be used as a bag of holding; the contents of the gemstone will not drop upon poofing but will drop upon shattering.
Cracking and Shattering. Cracking occurs when your gemstone is damaged by a destruction spell, attacked directly with bludgeoning damage, attacked after poofing, or you fail two death saving throws. While you are cracked, you incur two levels of exhaustion as well as cosmetic faults at DM discretion. Cracks may be repaired with the mending cantrip or jeweler’s tools, but cannot be performed by you. If you fail all three death saving throws, you are shattered into 2d4 Gem shards. Gem shards are still “alive” and as such cannot bring you back individually. If all the Gem shards are collected, the gemstone can be repaired by a professional jeweler and then restored with a spell such as resurrection.
Gem Glow. Your gemstone can produce a dim light to illuminate dark places. The light reaches up to 40 ft and is tinted in the gem’s color.
Imbue Intelligence. A poofed gemstone or gem shard may be built into a solid object, turning it magical. These objects are as intelligent and aware as the gems they used to be, but besides this how the gem influences the object is up to DM discretion. When the object is destroyed, the Gem is released and may reform.
Languages. You understand and can speak Common, abyssal, and the main language of a contract holder. 

Cursed Gem Traits

Cursed Gems share certain racial traits as a result of their infernal descent.

Ghostly Image

You can project from your Gem a holographic duplicate of yourself. During your turn, you may use your action to create a hologram, and may not create another until you complete a long rest. This hologram is functionally identical to the force that is created by the unseen servant spell, though it appears semitransparent and as your default form. You may use a bonus action to dismiss it. At 5th level, you may create a second hologram before resting, and at 5th level, you may create a third.

Ability Score Increase

You gain a +1 Bonus to Charisma and Dexterity.

Age

Cursed Gems mature at the same rate as humans but live about twice as long.

Alignment

Cursed Gems might not have an innate tendency toward evil, but many of them end up there, hence them being what they are today. Evil or not, an independent nature inclines many Cursed Gems toward a chaotic alignment.

Size

Cursed Gems can use available material to range in size, but their default size is medium

Speed

Your walking speed is determined by the material you are made from.

Darkvision

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Gem Physiology

You have immunity to poison and aging.

Demonic Heritage

You know the thaumaturgy cantrip. When you reach 3rd level, you can cast the Magic Weapon spell as a 2nd-level spell once with this trait and regain the ability to do so when you finish a long rest. When you reach 5th level, you can cast the darkness spell once with this trait and regain the ability to do so when you finish a long rest. Charisma is your spellcasting ability for these spells.

Languages

You can speak, read, and write Common, Abyssal and the main language of your contract holder.