On Tolkien Gateway (supposedly considered a good source for canon) in the entry for the Twelve Houses of the Gondolindrim, the following image is shown to represent the coat of arms:
Granted that most of the coat of arms are described by the name of the house, but had Tolkien ever described the design of the different coat of arms or were they simply made up?
The heraldry Twelve Houses of the Gondolindrim was described in the Book of Lost Tales II: The Fall of Gondolin.
The first house to be accounted is that of the King:
“Mighty was the array of the House of the king and their colours were white and gold and red, and their emblems the moon and the sun and the scarlet heart”
The Book of Lost Tales Part 2: The Fall of Gondolin
This suggest an accurate depiction of the House of the King in the image.
The House of the Wing and the House of the Mole were less clearly described:
“Lo! all these wore wings as it were of swans or gulls upon their helms, and the emblem of the White Wing was upon their shields. But the folk of Meglin were drawn up in the same place, and sable was their harness, and they bore no sign or emblem, but their round caps of steel were covered with moleskin”
The Book of Lost Tales Part 2: The Fall of Gondolin
The harnesses being "Sable" proves the depiction to be correct, as from Wikipedia:
In heraldry, sable (/ˈseɪbəl/) is the tincture black
However the Unfinished Tales seem to suggest that the Emblem of the House of the White Wing was cast on a "blue field" rather than white
“But the shield was of a shape strange to Tuor’s eyes, for it was long and tapering; and its field was blue, in the midst of which was wrought an emblem of a white swan’s wing.”
Unfinished Tales: The Coming of Tuor and the Fall of Gondolin
The House of the Swallow and of the Heavenly Arch were described as:
“Now the folk of the Swallow bore a fan of feathers on their helms, and they were arrayed in white and dark blue and in purple and black and showed an arrowhead on their shields.” [...]
“Every shield of that battalion was of the blue of the heavens and its boss a jewel built of seven gems, rubies and amethysts and sapphires, emeralds, chrysoprase, topaz, and amber.
The Book of Lost Tales Part 2: The Fall of Gondolin
Whether the House of the Swallow bore all 4 of their colours on their shield is unclear, however the House of the Heavenly Arch seemed to be clear and correctly drawn.
There too were the folk of the Pillar and of the Tower of Snow, and both these kindreds were marshalled by Penlod, tallest of Gnomes. There were those of the Tree, and they were a great house, and their raiment was green. They fought with iron-studded clubs or with slings, and their lord Galdor was held the most valiant of all the Gondothlim save Turgon alone. There stood the house of the Golden Flower who bare a rayed sun upon their shield...
The Book of Lost Tales Part 2: The Fall of Gondolin
The house of the Golden Flower seems to be correct, however that of the Pillar, the Tower of Snow and the Tree all seemed to have been made up (Besides that the Tower of the Tree wore green).
“Then came there from the south of the city the people of the Fountain, and Ecthelion was their lord, and silver and diamonds were their delight; and swords very long and bright and pale did they wield, and they went into battle to the music of flutes. Behind them came the host of the Harp, [...] They were dight with tassels of silver and tassels of gold, and a harp of silver shone in their blazonry upon a field of black...”
The Book of Lost Tales Part 2: The Fall of Gondolin
Again no description is given of the heraldry of the house of the Fountain, however there love of silver and diamonds would have influenced the design. The design of that of the house of the Harp seems to be accurate.
And finally the people of the house of the Hammer of Wrath:
“Now the last of the battalions was furnished by the folk of the Hammer of Wrath [...] The sign of this people was the Stricken Anvil, and a hammer that smiteth sparks about it was set on their shields, and red gold and black iron was their delight.”
The Book of Lost Tales Part 2: The Fall of Gondolin
In summary, the Houses of the King, the Wing, the Mole the Golden Flower, the Harp and the Hammer of Wrath seemed to be correctly drawn and reasonably accurate. The Houses of the Swallow and the Heavenly Arch seemed to be similar but not quite correct. Finally, the Houses of the Pillar, the Tower of Snow, the Tree and the Fountain seemed not to be given and based on the description of their names.
