You can see in dim light within 120 feet of yourself as if it were bright light and in darkness as if it were dim light. You discern colors in that darkness only as shades of gray.

Most Gems are lawful, the iron-fisted society of the Diamond Authority having instilled in them a soldier’s ability to follow orders unquestioningly and swiftly. Most are neutral as well, only used to following orders and obeying their rigid social structure instead of deciding the morality of their decisions beforehand.

On the other hand, those that have escaped the iron fist of the diamond authority are typically chaotic, ready to express themselves wildly after thousands of years of oppression.

Your body is a projection of hard light with mass. 
You have advantage on saving throws against poison and have resistance against poison damage. 
You are immune to diseases.
You do not need to eat, drink, or sleep but have the ability to do so.
You only need to enter a 4-hour trance to rest each night (or day) to absorb light from around you. This light may come from the sun or the moon, other natural sources like lanterns and fire, or magical sources like the Light Spell.
If you do not meet this requirement in a 48 hour period, your body visibly dulls in color and bags form under your eyes, giving you a sickly appearance. You then have a disadvantage on all constitution saving throws and obtain a level of exhaustion.  
If you do not meet this requirement after a total of 72 hours, your body retreats into your gem until it meets its light requirement. 

While all gems are medium, their heigh varies wildly depending on their caste. Warrior gems tend to be tall and bulky, whereas servants can be tall or short and are usually thin. Technicians tend to be about average height and aren't particularly thin or bulky.

Gems do not age, and cannot die unless the gem at the core of their being is shattered.  They burst from the ground fully formed, knowing exactly what their purpose is.

Your gem is the core of your being; comparable to the heart, soul, consciousness, etc, of other races. Your 'body' is simply a projection of hard light with mass. When you would normally take fatal damage, you retreat into your gemstone. Your gemstone has 2d4 + Your Con. Modifier Hitpoints.
Your gemstone can only be healed by magical means and does not regenerate with your projected body. A healing spell can target either your body or your gemstone. 

You are considered unconscious while inside your gemstone but may communicate telepathically if someone else initiates the communication. You can only communicate in simple sentences and ideas, however.

For every round of combat, you may make a 10 DC wisdom saving throw to reform with 1 Hit Point. Each time you use this feature after the first the DC increases by 5. This DC is reset back to 10 after a Long Rest

If your gemstone HP falls to half of its maximum, it becomes cracked. You then have disadvantage on all Wisdom and Intelligence ability checks and throws. Your projection also begins to noticeably glitch and deform which may disturb humanoids.

When your gemstone hit points reach zero, its shatters completely and you die. The revivify spell or stronger can restore your gemstone for up to half of its maximum hit points.

You can summon a weapon (decided at character creation) from your gemstone as a bonus action. You may have up to two of them at a time. 
The weapon is weightless and made of hard light like your body. You are an expert in its use and may re-summon it as a bonus action from your gem if it is dropped or destroyed. The weapon or weapon pieces being replaced disappears in a small burst of dull light.  It has +1 and is magical for the purpose of overcoming resistances. 

You have disadvantage on all attack rolls that are made with anything other than this weapon.

You can "bubble" any object who's size is small or smaller. The object is suspended inside of a bubble matching your gemstone's color and can be transported to wherever you are currently living by pressing the top of it. The bubble can be popped, releasing whatever was inside, and any gems that are bubbled are unable to regenerate.

You can speak, read, and write Common and Celestial. Celestial sounds beautiful yet alien in nature, and is often used by clerics.

Fusion Gems are the product of multiple Gems (or half-Gems, who can fuse with both Gems and humans). Fusions are formed when the participants are emotionally harmonious with each other. This state can be spontaneous, but it is usually achieved deliberately through a synchronized dance.