Talking about courage of the colossus, the mastery keystone.
Since she has a built in shield... she could benefit from Runic Armor, then stacking Colossus ontop of that from her Qs knock up, or her Stun.
Which this would only happen when your team need someone offtanky, which my build would be >
with last item being either 
depending on what was needed.
That way i get my 40% cdr, but plenty of armor/mr/hp, while keeping AD up. Whats yalls thoughts?
Not anymore.
It used to be when it was broken (i mean every single champ with atleast 1 hard CC could use it back then), since the nerfs it now isnt.
CoC is a good option on riven but not as great as it was
It's completely useless against tanks and generally inferior to FoB (which you should take in 99% of all matchups). If you want a more utility heavy mastery I suggest you try Stormraiders Surge which should help you prevent more damage than the CoC shield. Overall it's good for flashing into 5 people but it's just so lacking in laningphase that I wouldn't recommend taking it.
I think the main problem is the buildup to CoC isn't great on her. Slow resistance or armor? meh, most of her movement comes from abilities, not her actual ms, 45% cdr would probably make her move faster, and she has nothing that scales with ar/mr.
lower cdr on summoners, high regen when low hp, bonus res for 2 seconds? meh, she's not super reliant on summoners, doesn't really need super regen as she has a shield/stun to bait people while low hp, and she usually takes more than 2 seconds to pull off her combo, and they're likely stunned for the 1st 2 seconds anyway, so extra res. doesn't do much.
runic armor, hell yeah she loves that...but you can get that going 0/18/12 or 18/0/12, no need to invest in last 2 tiers of resolve for it.
I think whatever benefit you'd get from CoC, you'd do better with the 5% extra cdr or 7% armor pen, plus warlords, fervor, stormraiders, and of course thunderlords are all going to be good on her.
And get your off-tank stats elsewhere
like 
etc
tl:dr- she'll get more out of going 'normal' mastery lines+there's plenty of in-game+adaptable options for an off-tank to build
Seen it with bc and titanic build
Nope. Why go defensive when you can play another champion instead who is better suited for that role? Riven is all about aggression at early/mid pressure and making best plays.
If you are one of those players that always pick Riven even if your team badly needed a tank...why not? You could do a build that is slightly more tanky than usual and use the shield to initiate fights.
I think CoC is a good option if you need an off tank. I use CoC on Jax when I see we have no tank and ir procs quite well in early trades with e as a top laner. I think the only tradeoff is you don't get the typical TLD procs that are better synergy (imo) with her kit as a damage oppressive champ.
I tried this actually. I tried playing a game where I had CoC, and I was rushing Sunfire.
I was against Garen top with Thunderlord's Decree and he destroyed me.
Then again, I'm not an experienced Riven player so I suppose it was fair that I got rekt, but I'm just sharing the results with you.
