The Diamond Heirloom Blanket is a 20-square throw designed to be made slowly and kept for a long time.

Each square grows from a center ring outward through 12 rounds, building a solid double crochet frame in the first four rounds before opening into a diamond-ridge lattice in rounds 5 through 11. The lattice widens symmetrically from the center of each side — one additional repeat per round — until the diamond fills the square completely. A final round of single crochet closes and frames each motif.

The squares are joined with a visible single crochet ridge seam, a deliberate design choice: the raised seam catches light and gives the finished blanket structure and presence. A 5-round border finishes the edge — two rounds of single crochet, a double crochet mesh worked in two layered rounds, and a final round of single crochet.

Materials
Lion Brand 24/7 Cotton DK (100% mercerized cotton; 186 yd / 170 m per 100 g skein) — 10 skeins total (approx. 1,860 yards). Shown in Ecru and White.
3.5 mm (US F/5) hook, or size needed to obtain gauge.
Tapestry needle, rustproof blocking pins, steam iron or spray bottle.

Gauge
Each finished square measures 10½ in (26.5 cm) after steam blocking. Gauge is important — block and measure your first square before continuing.

Finished size
Approx. 48 in × 58 in (122 cm × 147 cm) — a generous throw.

Techniques used
Double crochet, single crochet, slip stitch, chain spaces, front post stitches, corner shells, single crochet join, steam blocking. All rounds are worked on the right side; no turning between rounds.

A note on the diamond ridge
Beginning in Round 5, a ch-1, skip sequence introduces the central openwork. Each subsequent round adds one more repeat on either side, widening the diamond outward. The starting point shifts slightly each round — this is intentional, not an error. Trust the shift.

Pattern notes
Written in US crochet terms. Stitch counts given at the end of each round. Block all 20 squares before joining for the cleanest seams and most accurate finished size. Care: machine wash cold, gentle cycle; tumble dry low or lay flat. Cotton softens with use.