The Gigantic Crystal (Cyan) decoration can be dropped from Constable's Stronghold Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Constable's Stronghold Pack, include a random selection of items from the Constable's Stronghold collection of items. You aren't guaranteed to get a Gigantic Crystal (Cyan) decoration from a Constable's Stronghold Pack, but it is one of the few ways to get a Gigantic Crystal (Cyan) deco apart from buying it on the GTN from another player.
If you're looking to get a Gigantic Crystal (Cyan), the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.
Contributors: Thank you Danata [Jung Ma] for allowing his Stronghold items to be shared!
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