Elven Technology - how did they do it?

[Legolas]

Apparently he was just a ship builder, and some elves of his settling mined the sea for pearls, since he gave some to Thingol who passed them on to the Dwarves of Belegost. There are numerous mentions of Cirdan and his people building ships throughout the War of the Jewels.

The Silmarillion:

But the realm of Nargothrond extended also west of Narog to the River Nenning, that reached the sea at Eglarest; and Finrod became the overlord of all the Elves of Beleriand between Sirion and the sea, save only in the Falas. There dwelt those of the Sindar who still loved ships, and Círdan the shipbuilder was their lord; but between Círdan and Finrod there was friendship and alliance, and with the aid of the Noldor the havens of Brithombar and Eglarest were built anew.

For Melian taught them much that they were eager to learn, and Thingol rewarded them with many fair pearls. These Círdan gave to him, for they were got in great number in the shallow waters about the Isle of Balar; but the Naugrim had not before seen their like, and they held them dear. One there was as great as a dove's egg, and its sheen was as starlight on the foam of the sea; Nimphelos it was named, and the chieftain of the Dwarves of Belegost prized it above a mountain of wealth.