A collection of titanium Patek Philippe watches that were previously unknown to collectors will come up for auction with Sotheby's New York on June 10, starring a unique Sky Moon Tourbillon.

This is the only known Sky Moon Tourbillon made from titanium, a lightweight and ultra durable metal

The Sky Moon Tourbillon is the most sought after watch from Patek Philippe, which reigns supreme in the collector watch market. With an undocumented reference, 5001T, and the only known example, it is expected to sell for $1m-1.5m.

The watches all hail from a private collection of 12 timepieces, each of which incorporates rare materials and  designs - suggesting they originate from one of Patek Philippe's most prized customers.

The Sky Moon Tourbillon, made in 2000, has a basket weave motif in place of the brand's usual Calatrava crosses.

Until the announcement of the sale, there were just four watches in the world made from titanium by Patek Philippe, so expect strong bids come sale day.

Patek Philippe's first split seconds chronograph

The collection also houses one of the most important wristwatches ever produced by Patek Philippe: the 1923 yellow gold Officier, which was the first split-seconds chronograph the company made.

Also the only split-seconds chronograph to feature a white enamel dial, it's valued at $800,000-1.2m.

For those looking to enter the world of watch collecting, Paul Fraser Collectibles has a classic Longines Olympic series watch for sale.