Starts with a slight tie-in between 3X06 and 3X07 but the rest is canon complacent until stated otherwise.
Note, There is platonic Clizzy here.
"Izzy I still can't believe you told that guy we were jewelers!" Alec laughed.
Isabelle groaned and lightly smacked her brother's arm. "Shut it Alec! I went with his assumptions after I said I made my bracelet..." Alec smiled.
"I'm not meaning anything bad about it. It's just funny. I'm waiting on his reaction to the truth."
"I can tell you now that won't happen. He won't answer my texts. And I'm okay with that." She replied.
Alec smiled and hugged her. "Whatever makes you happy Izzy, you know I'm here if you need me. Even if I know you can handle yourself." She grinned and buried her face in his shoulder.
"Thanks big bro..." She replied.
"Though quick question, the ruby in your bracelet, is that the same one from your necklace Magnus gave you?" Alec asked looking down at her bracelet.
Isabelle grinned and nodded. "It is. I even welded pieces of the chain to the main band." Alec smirked and let go of her wrist.
"I see why the mundane believed it. This is really nice. What's next, a necklace? Engagement rings?" He joked. Izzy opened her mouth but a knock sounded, which sounded more like metal hitting wood that both siblings recognized immediately.
"Magnus?" Alec called. The beautiful warlock opened the door, and though he himself looked tired, his eyes showed panic and sadness.
Alec was immediately alert. "Babe what's wrong?"
Magnus sighed. "Alexander, you need to hear this..."
~Two years later~
Everything had settled, but something stuck out in Isabelle's mind. The way Alec had joked about her making engagement rings that night she brought the mundane to a family dinner.
When he had told her about looking into the family vault for a special ring for Magnus it clicked in her head.
She'd spend hours drawing out ideas, making sure Alec never saw them when he swung by her desk, but nothing really stuck until Alec mentioned something about the family sigil, which was fire, and how the sunrise 'looked like glowing embers in Magnus's gorgeous eyes'.
She might've giggled and patted her lovestruck brother on the back for that utterly romantic line.
"Let me go with you next time you go." She had offered. Alec just smiled and nodded his head.
Which is how she found herself meandering through different pieces of gawdy jewelry that nobody thought to toss when she saw it.
A simple band that was tarnished to hell and the stone murky and dull. She swiped the ring and slid it into her pocket when she saw a stone near the back that looked to be a gold topaz that needed to be polished and cut better but otherwise looked decent. She took the gem as Alec turned the corner.
"Find something Izzy?" He asked. Izzy shook her head.
"Sadly no. Everything here is so gawdy and looks like something Imogen would've worn."
Alec sighed. "Lightwood ring it is..."
"Save that for the wedding big bro. Plus there are plenty of options for rings online." She tried.
"Except I really was hoping for asking him on the day of our first kiss." Alec explained.
Izzy internally sighed, this was gonna be difficult.
"Thankfully it's not till next month so I have time...I just want to do it right Iz..." He rambled on as Isabelle put her hands on his shoulders.
"Breathe big bro. You're right, you do have time, and honestly, you could propose with a ring from a gumball machine and he'd wear it with absolute pride, green finger and all." She assured him, who laughed.
"You're right sis...thanks. Though I should still look into something nice. He deserves everything nice in this world..." He replied, a dreamy look in his hazel eyes.
Isabelle rolled her eyes affectionately. "Good lord you lovesick puppy..."
Alec gave her a pointed look. "Shut up and let's go. I need to get back home in a few minutes."
Izzy smiled, the vault closing behind them and as Alec left for the day, she looked over the ring and stone and set to work polishing away the tarnish and grime on the band and by the time she was done it was nearly dawn.
"Isabelle?" A voice called. Izzy looked up to see Clary, her hair falling like a bright red waterfall down her back.
Izzy gave a tired smile. "Hey Clary. What's up?"
"I just got back from patrol. Quiet night. It looks to me like you've been working on a new project? What is it?" She asked as she sat down next to her.
Isabelle sighed. "You have to swear not to tell anyone about this, or at least wait till I'm finished with it." Clary nodded. Izzy looked around to make sure nobody was listening before lowering her voice.
"Alec had made a joke about me making an engagement ring and honestly for a while I forgot about it, but when he brought up that he wants to propose to Magnus, and the fact that he's having a hard time to find a good ring, I decided to make one." She explained.
Clary's eyes went wide. "Does Alec know about this?"
Izzy shook her head. "Not yet. I'm planning to wait till I'm done with cleaning up the band and stone to say something. But I looked at his family ring that he wanted to propose with originally and the way the lines on the rings line up, it's gorgeous and it looked like they were a set but this got tossed in the vault to tarnish. It's kinda sad honestly..." She rambled. Clary smiled.
"This is amazing Izzy. Though how are you going to ensure that it fits?" She questioned.
"I...haven't gotten that far yet...I'm hoping Alec knows..." Izzy replied.
"Hoping I'd know what?" Alec questioned. Both women jumped at the sound, and Isabelle was thankful the ring and stone were covered by the cloth.
"Hey Alec! What are you doing here? I thought you'd be home with Magnus." Clary greeted.
Alec frowned slightly. "Early morning client..." He grumbled, and Isabelle knew that he was grumpy from his time with Magnus being cut short.
"But what were you two talking about?" He questioned again. Clary looked at Izzy, who thankfully had a few plans.
Which were currently escaping her tired mind.
"We were hoping you'd know where Magnus gets some of his makeup! The stuff he has is life proof pretty much. Plus it's bright and bold. It's awesome." Isabelle lied quickly, a sweet smile gracing her red lips, hoping this would throw off her brother.
Alec's eyes narrowed. "You know that doesn't work on me Izzy. Tell me the truth please?"
A moment passed before Isabelle sighed. "It's a surprise big bro. I'll tell you, just let me finish it."
Alec nodded seeming to have accepted this. "Alright. Though get some sleep ok Iz? I can tell you've been up all night working again."
The girls watched as Alec trudged to his office before breathing a sigh of relief. "That was close..." Clary whispered.
Isabelle nodded. "That was also crap lying on my end...I shouldn't try that when I'm tired..."
At that, she yawned and rubbed at her already ruined makeup. Clary smiled at her before standing.
"Come on. My room is closer and we're both dead on our feet." She stated as she extended her hand.
Izzy smiled as she took it. A bed sounded amazing, no matter who's it was. The pair made it to Clary's room, where they were out within minutes of their heads hitting the pillows.
As the days turned to weeks, Isabelle made quick work of the topaz and made it shine the way it should. The band itself, which underneath the grime and dust was white gold and much easier to manipulate than she thought, was polished and ready for the stone to be set. She had been examining her handiwork when a voice called out.
"Whoever gets that is one very lucky person with that kind of craftsmanship. You have a talent Isabelle."
She looked up to see Magnus leaning against her desk with a smile.
"Hey Magnus. Looking for Alec?" She guessed.
He smirked. "I know where he is, I just happened to see you. May I see the piece you're working on?"
She nodded and handed the band to him.
He quietly examined the band, taking note of the way the lines on the band moved. "What stone do you plan to set into it? And who might this be for?"
You of course! Alec is gonna propose with this ring! She screamed internally.
"A couple from another institute saw me working on my bracelet one day and asked if I could make this for them. They brought me the materials to make it along with several stones, but I can't decide which." She explained. It was absolute bullshit but it needed to be convincing. Though this gave her a perfect opportunity.
"Try it on actually? The guy that I'm making this for has a similar build to you and since I won't see him till much later, I want to make sure it fits." She suggested.
Magnus quirked an eyebrow. "Are you sure Isabelle?" She nodded.
"Go for it."
Magnus shrugged and slid the ring on his left ring finger. It fit perfectly, and Izzy was amazed at how the ring went with the others on his hand.
Damn I'm good... She thought.
"Fits perfectly Izzy. This is incredible!" Magnus praised.
"What is babe?" Alec asked. Izzy internally gulped.
Magnus smiled. "Your sister. She made this for a couple at another institute. The design is elegant and very eye-catching." He pulled the ring off and showed Alec, who looked it over before handing it to Isabelle. She knew that look in his eyes as she she saw the pieces click together in his head.
Thankfully she knew that he knew better.
"It's beautiful Iz." He agreed.
Magnus grinned and took his hand. "Well come on Alexander. I've heard of a new restaurant we need to try. It opened recently and I'd like to make sure we're not late to our reservation."
Alec nodded but looked back at Isabelle with his "We're talking later..." glare.
She might be in trouble.
"You were lying to Magnus about that ring Izzy." Alec stated, his arms crossed as he stared at his sister.
Isabelle cringed. "Busted. I needed a means to make sure the band fit and well..."
"Why would you... Oh." Alec realized as she gave him a soft knowing smile.
Alec sighed. "So when were you going to tell me you were making Magnus's engagement ring?" He inquired.
Isabelle sighed. "I actually was planning to tell you today but you had been caught in those meetings with the clave."
Alec sighed as he looked down at the ring in his hand. "What were you going to set into this?"
"A golden topaz. I was going to let you set the stone and engrave something on the inside of the band. Wanna see?" She explained. Alec nodded and she opened a tiny box revealing a beautiful topaz. It was a simple, circular cut but the way it gleamed in the light made it far more.
Isabelle smiled at her brother's awestruck gaze. "We can set it tonight if you want big bro."
The awestruck but enthusiastic nod didn't go unnoticed as she set Alec down at her desk and moved the box to where the ring was sitting and with a pair of tweezers he gently picked up the stone and slowly set it against the opening. The gem settled perfectly in the prongs that were there.
"I...um..." Alec stammered, flabbergasted that he was doing this at all.
Isabelle smiled. "Now you need to tighten the prongs so the stone doesn't fall out. Here." She explained as she used a second set to press two opposite prongs together. Alec nodded and did the same thing to the other set before picking it up.
"Wow..." He gasped, tears pricking in his eyes as he pulled his sister into a tight hug. "Thank you."
Isabelle smiled. "You're welcome big bro."
"Seriously Izzy, you are the best sister I could ask for. This is perfect!" Alec exclaimed. She grinned and buried her face in his shoulder.
"I'm glad you like it. Now I'm gonna set up the machine to engrave whatever you want into the metal." She explained as she let go.
Alec nodded as he grabbed a sheet of paper. "Thank you Izzy. Do you think this will fit?" He asked as he wrote down something. She looked it over in confusion.
"What does it mean?"
"Not going anywhere. I...there's a possibility of me becoming immortal. And I want to take that chance if it means Magnus never loses another person he loves. I want forever with him Izzy..."
Isabelle was momentarily stunned but she saw that devoted look in his eyes and she knew there was no changing his mind.
"Big bro...I told you before, if it's what you want, what makes you happy, then do it. If you want to be immortal with Magnus then do it." She told him. Alec hugged her tightly.
"Thank you Izzy...you really are the best..." He whispered as he kissed her hair before she pulled away to finish up the machine.
"How do you plan to do it?" She asked.
Alec grinned. "Romantic picnic and a walk through central park. I'm keeping my fingers crossed for decent weather so I can propose under the stars."
Isabelle couldn't help herself as she giggled and ruffled her brother's hair. "You hopeless romantic...but the idea is perfect."
Alec laughed. "I just hope I can actually say what I want to say though..."
Isabelle sighed. "Alec, stop overthinking. Just say what you feel. You love Magnus, and you want forever with him. The fancy eloquent words are well and good, but sometimes just blurting out what you feel is a better option." She told him. Alec took a deep breath before smiling.
"Thanks Izzy. So when is this supposed to be done?"
She couldn't help but smile at her excited older brother. "This will be done in the morning and you'll be set. All you need now is a box."
When Isabelle checked her phone a few days later, her phone had several texts, including a picture message from both Alec and Magnus. She checked the pictures first to reveal Alec and Magnus's hands, Magnus's on top showing off the ring she made, and the way it sparkled in the early morning sun looked amazing.
The caption attached said "Really Isabelle, if you wanted to know my ring size you could've asked. ;) Thank you!"
She laughed at that and sent "Why would I ask? I'm not the one you're marrying. Plus it would've ruined the surprise. XD" before looking over Alec's texts, ranging from nervousness to an overtly excited "HE SAID YES!" with several hearts and smily faces.
She couldn't help but giggle and grin over his happiness. She shot back a reply of "I'm helping plan the wedding big bro!"
She then tagged on as a joke "Am I making your wedding rings too?"
"No but will you teach me?" A reply came back. Isabelle laughed and knew this was gonna be a thing for her from now on.
"I've got you big bro. I've got you."
Let me say this once: I WILL WRITE THE PROPOSAL WHEN I HAVE TIME! And I have no idea what to write with gorgeous love speeches or things like that...