Your gem has a health pool separate to the golem, equal to your Constitution modifier +3. If your gem ever reaches 0 health points, you have the option to activate the Swansong ability before your soul is forcefully ejected from the golem and you are considered dead. You may persist as a spirit at the DM's digression until you can find a new host.

Your gem can be hit if you ever receive more than 50% of your health points in a single attack or are struck with a critical hit. Any creature with deep knowledge of a Vestige Golem's function can directly target the crystal, avoiding the previously said conditions.

If your gem is ever struck, roll a Constitution saving throw (DC 16). Upon failure, you take a point of exhaustion and receive 1.5x the damage the attack would usually have done as the magic keeping you within the Vestige Golem surges and pulses through your vessel to keep you safe. If your gem is struck and the increased damage would do more than your maximum health, your soul gem is shattered instantly and Swansong is activated regardless of the player's wishes.

Your stone form gives you resistances to fire and conventional weaponry  (piercing, bludgeoning, slashing). Unfortunately, your gem is quite sensitive to electricity, giving you vulnerability to lightning damage.

Your tough stone body and magical reinforcement gives you a +2 to AC.

You only receive 0.5x the amount of healing you would usually receive from traditional healing spells. when the Mending spell (or spells of similar "repairing" function) is cast on you, roll 2 of your hit-die at no cost and add the spellcasting modifier of the caster to the result, heal this amount. 

When falling to 0 hit points and beginning death saves, your spirit within your crystal remains conscious but unable to move the golem it controls due the damage. Your body will remain stiff and uncontrollable until you receive healing. Upon succeeding 3 death saves, you do not regain control of your body, you must receive healing of any kind before being able to move again.

At any time you wish or upon failing 3 death saves (or at any point you choose when rolling them) your soul is ejected from your soul gem and the remaining magical energy begins a destructive meltdown of your Golem body. The Vestige Golem will begin to shake and pulse with magical energy, pushing all creatures, projectiles and objects that touch it away 20ft and knocking creatures prone if they fail DC 12 Dexterity saving throw. After 4 turns/20 seconds, the Vestige Golem explodes in a 50ft cube, dealing 3d10 fire damage, 1d10 thunder damage and 1d8 bludgeoning damage due to the violent magical energy, shockwave and shrapnel.