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The only sure way of determining the doneness is to test the internal temperature with an instant read thermometer inserted precisely into the center of the meat at a 45 degree angle to the flat surface. This will ensure a correct reading of the internal temperature. Don't worry about losing any juices---it's only one small hole you're making. The pictures below represent what each doneness means to a chef.
Black and Blue
Internal Temp. 70° F --- Black and Blue---Seared black on outside, ice cold raw center and will be slippery soft in texture.
Internal Temp. 100° F --- Very Rare---Hot on the outside, raw on the inside and will be sort of wobbly.
Internal Temp. 120° F --- Rare---Red cool to warm center and will be soft and spongy.
Internal Temp. 126° F --- Medium Rare---Red warm center and will have a springy firmness.
Internal Temp. 135° F --- Medium---Hot pink center and will have a less springy firmness than medium rare.
Internal Temp. 145° F --- Medium Well---Slight color, cooked throughout and will feel firm.
Internal Temp. 160° F --- Well Done---No color Left and will feel very firm and unyielding.