Starcrystals are crafting materials that are dropped by the final bosses of each Heroic Mode area, and are used to craft the various Heroic Mode armors. Initially these were only the Crystal Finmail (Water), Crystal WingGuard (Air), Crystal Geoplate (Earth), Crystal Devilplate (Spirit) and Crystal Dragonmail (Fire). However, most of the "top-tier" (strongest) Epic Armors have now been included in Heroic Mode.
In order to collect enough materials to craft the Heroic armor, you must complete each of the Heroic stages 8 times (receiving 1 Starcrystal Fragment each time), resulting in a single Starcrystal. Prior to November 21st, 2016, you needed to collect 8 total Starcrystals, however now you only have to collect 6 from the first 6 stages (Relic Ruins, Thieves Cove, Skeletons Tomb, Flaming Wildwood, Cold Rock Crags, and Faerie Forest).
To craft the Plus (+) version of the armor, you must upgrade the Regular version of the armor to level 35 (or higher) and then collect an additional 6 Starcrystals from the next 6 stages (7-12, Writhing Cascades, Guardian's Crossing, Sparkling Steppes, Blazing Tides. Caballero Desert and Sunken Carrack).
The last 4 stages (Misty Marsh, Zephyr Plateau, Haunted Citadel and Kingdom of Darkness) are much more difficult than previously, however they now provide Shadowforged Armor crafting materials as the rewards for each stage (each only needs to be cleared once).
Starting on November 21st, 2016, Heroic Mode returned with the initial "revamp" (shown as "Beta") allowing players to craft the Heroic armor with only 6 Starcrystals instead of 8, and on December 15th, 2016, Heroic Mode was revamped again, and now Dragonforged Armor can be crafted in addition to Shadowforged Armor crafting materials being the reward for the last 4 stages.
The rewards differ depending on the Heroic Mode; for instance, the first (the first partial "revamp" on November 21st, 2016) included Silvanus Gems (to craft the Silvanus Stonemail), the second (after the revamp in mid-December, 2016) included Aeolian Crests (to craft the Aeolian Abomination, and the third (so far) includes Chrono Cores (to craft the Chronos Regalia).
For further details on Heroic Mode, please go here.