Spoiler: Reply to above post
Likewise, the attack bonus is +5. That's 2 more than the target attack bonus for the DPR, so you shift Atk CR up by one.
You can't just go throwing in random CR bonuses for abilities. Especially in this case, because the 'Klingon toughness' doesn't affect the overall CR at all. All it adds is one save and one damage resistance (the regeneration is already taken into account above). Neither save bonuses nor resistances matter until you have three or more of either of them, and then they just modify the effective HP and AC.
Okay, I've just watched the entirety of Steven Universe this weekend, so I think I'll start with my favourite gem, Pearl. I'll be aiming for a CR of 3 or 4.
Holo-Pearl = Mirror Image, right?
Medium humanoid (gem), lawful neutral
Armour Class 14
Hit Points 78 (12d8 + 24)
Speed 30 ft.
STR DEX CON INT WIS CHA 12 (+1) 18 (+4) 14 (+2) 19 (+4) 7 (-2) 6 (-2)
Saving Throws Con +4, Wis +0
Skills History +8, Investigation +6
Senses passive Perception 8
Challenge 3 (700 XP)
Do it for Her. If Pearl can see an ally, she has advantage on Wisdom saving throws and is immune to the Frightened condition.
Innate Spellcasting. Pearl's spellcasting ability is Intelligence (spell save DC 14). She can innately cast the following spells, using her gem as a focus.
At will: Eldritch Blast, Minor Illusion
1/day: Mirror Image
Magical Weapons. Pearl's weapon attacks are magical. As a free action on her turn, Pearl can switch her weapon between its sword-form (which is one-handed) and spear-form (which is two-handed).
Multiattack. Pearl makes three melee weapon attacks.
Spear. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) slashing damage.
Sword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage.
Parry. Pearl adds 2 to her AC against one melee attack that would hit her. To do so, she must see the attacker and be wielding a one-handed melee weapon.
Spoiler: NotesI've statted the 'spear' as if it were a finesse longsword/battleaxe, because that's a fair representation of how she wields it.
Def CR: 78HP = CR1, AC=14+1 (parry)=15, final Def CR: 2
Atk CR: 27DPR = CR4, Atk bonus = +6, final Atk CR: 4
Overall CR: (2+4)/2=3.